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Your Timeline (July 13-15)

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Sleep/Wake Schedule

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Available Study Hours

Total usable study time: ~19-20 hours

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Subjects Remaining

1. Parasitology
2. Immunology
3. Bacteriology

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Study Allocation

Session 1 - Tonight (11:53 PM → 11:00 AM) = ~11 hours

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Session 2 - Evening (7:00 PM → 2:00 AM) = ~7 hours

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Exam Morning (6:30 AM → 8:45 AM) = ~1.5 hrs

• Quick notes scan only - no new topics
• Focus on mnemonics, classifications, and organisms you're shaky on

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Priority Topics (High-Yield for LS4)

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Key Tips

• Don't re-read - revise. Use short notes, mnemonics, previous year questions
• Parasitology and Immunology are more memory-based - do them first when your brain is fresh
• Bacteriology is vast - focus on high-yield organisms only, skip rare ones
• Set alarms - sleep deprivation kills recall more than skipping one topic

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Corrected Timeline

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18 Hours Split Across 3 Subjects

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Session 1 - Tonight (Now → 11:00 AM)

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Session 2 - After Waking (7:00 PM → ~3:00 AM)

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Exam Morning (6:30 - 8:45 AM)

• Mnemonics only
• Classifications, organism comparisons
• No new topics

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High-Yield Focus Per Subject

• Malaria - Plasmodium species, lifecycle, cerebral malaria
• Leishmania types (cutaneous/visceral)
• Entamoeba histolytica vs dispar
• Taenia solium vs saginata, Echinococcus
• Toxoplasma, Cryptosporidium, Giardia

• Hypersensitivity I-IV with examples
• Complement (classical vs alternate vs lectin)
• T cell vs B cell differentiation, CD markers
• MHC I vs II
• Primary immunodeficiencies (DiGeorge, SCID, Bruton's)
• Vaccines - live vs killed

• Gram +ve: Staph, Strep, Clostridium, Bacillus, Listeria
• Gram -ve: Neisseria, E.coli, Salmonella, Shigella, Vibrio, H.pylori
• Mycobacterium TB - full detail
• Spirochetes: Treponema, Leptospira, Borrelia
• Toxins: exotoxin vs endotoxin table
• Culture media (high yield for exams)

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Bacteriology Sprint: 4:00 AM → 9:00 AM = 5 Hours

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5-Hour Bacteriology Block Plan

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What to Focus ON vs SKIP

✅ DO cover (exam favorites)

• Staph aureus - toxins (TSST, exfoliatin, coagulase), MRSA
• Strep - classification (alpha/beta/gamma), Lancefield groups, Rheumatic fever
• Clostridium - tetani, perfringens, botulinum, difficile (mechanisms!)
• Salmonella vs Shigella differences (key table)
• Mycobacterium TB - cord factor, Ghon complex, PPD, culture (LJ medium)
• Neisseria - gonorrhoeae vs meningitidis differences
• Vibrio cholerae - rice water stool, El Tor biotype
• Treponema pallidum - VDRL vs TPHA
• Rickettsia - Weil-Felix reaction
• Chlamydia - types (A-C trachoma, D-K genital, L1-L3 LGV)
• Exotoxin vs Endotoxin comparison table - always in exams

❌ SKIP or skim only

• Rare organisms (Brucella, Francisella, Pasteurella)
• Detailed biochemical tests
• Long historical context
• Antibiotic mechanisms (unless specifically in your syllabus)

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Must-Know Quick Tables

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Your LS4 Syllabus - What's Actually in the Exam

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✅ VIROLOGY - DONE

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Remaining Topics Breakdown

🦠 BACTERIOLOGY (Clinical Microbiology - Weeks 2-9)

🦟 PARASITOLOGY (Weeks 13-14)

🛡️ IMMUNOLOGY (Weeks 1-15)

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Revised 18-Hour Plan (You're on Bacteriology now at 4 AM)

Session 1 - Tonight (4:00 AM → 11:00 AM) = 7 hrs

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Session 2 - Evening (7:00 PM → ~3:00 AM) = 8 hrs

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Exam Morning July 15 (6:30 AM - 8:45 AM)

• Scan your key comparison tables only
• No new topics

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Key Point

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🦠 STREPTOCOCCUS & ENTEROCOCCUS

Exam-Focused Teaching | ~45 minutes

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STEP 1: THE BIG PICTURE - Classification

Two Classification Systems - Know BOTH:

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STEP 2: GROUP A STREP (S. pyogenes) - The Most Important

Virulence Factors (HIGH YIELD)

Mnemonic for GAS toxins: "Some Sinister Killers Hide, Dodging Poison" Streptolysin, Streptokinase, Hyaluronidase, DNase, Pyrogenic exotoxins

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Clinical Diseases of GAS

A) PHARYNGITIS ("Strep Throat")

• Sudden onset fever, sore throat, NO cough (cough = viral)
• Tonsillar exudate, tender anterior cervical lymph nodes
• Centor Score (1 point each): Fever + No cough + Tender cervical nodes + Tonsillar exudate
  • 0-1 points → no test, no antibiotics
  • 2-3 points → rapid antigen test
  • 4 points → treat empirically
• Treatment: Penicillin V (first line) or amoxicillin

B) SCARLET FEVER

• Pharyngitis + diffuse sandpaper-like rash (blanches on pressure)
• Strawberry tongue (white initially, then red)
• Pastia's lines - rash accentuated in skin folds (axilla, groin)
• Caused by pyrogenic exotoxin (SPE) - superantigen
• Circumoral pallor (pale around mouth)

C) SKIN INFECTIONS

D) TOXIC SHOCK SYNDROME (Streptococcal TSS)

• Hypotension + multi-organ failure
• Often from necrotizing fasciitis/wound infection
• Caused by SPE superantigens (vs Staph TSS caused by TSST-1)

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Post-Streptococcal Sequelae (VERY HIGH YIELD)

🔥 RHEUMATIC FEVER

• Occurs 2-4 weeks after GAS pharyngitis (NOT skin infection)
• Mechanism: molecular mimicry - M protein antibodies cross-react with cardiac tissue
• Jones Criteria - need 2 Major OR 1 Major + 2 Minor:

Mnemonic: JONES = Joints, cOrditis, chNoreA (Chorea), Nodules, Erythema marginatum, Supporting (ASO)

• Treatment: Penicillin + aspirin (arthritis) + steroids (carditis)
• Prophylaxis: Monthly Benzathine Penicillin G injections for years (prevents recurrence)

🔥 POST-STREPTOCOCCAL GLOMERULONEPHRITIS (PSGN)

• Occurs 1-3 weeks after pharyngitis OR 3-6 weeks after skin infection
• Immune complex deposition in glomeruli → nephritic syndrome
• Hematuria, proteinuria, hypertension, oliguria
• Low complement (C3↓), elevated ASO titre
• Spontaneously resolves - antibiotics do NOT prevent it (unlike RF)

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STEP 3: GROUP B STREP (S. agalactiae)

• Normal flora of vagina (~25% women colonized)
• CAMP test positive - key lab ID
• Main victims: Neonates + pregnant women + elderly + immunocompromised

• Prevention: Screen all pregnant women at 35-37 weeks; give IV Penicillin during labor if positive

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STEP 4: S. PNEUMONIAE (Pneumococcus) - α-hemolytic, No Lancefield group

• Lancet-shaped diplococci (pairs, not chains)
• Quellung reaction - capsule swells with specific antisera (used for ID)
• Optochin sensitive + bile soluble (distinguishes from viridans)
• Polysaccharide capsule = main virulence factor; anti-phagocytic

• Treatment: Penicillin (if sensitive), Ceftriaxone
• Vaccines: PCV13 (conjugate, children) + PPSV23 (polysaccharide, adults/elderly)

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STEP 5: VIRIDANS STREPTOCOCCI - α-hemolytic

• Part of normal oral flora
• Optochin resistant + bile insoluble (vs S. pneumoniae)
• S. mutans → dental caries
• S. mitis, S. sanguis → Subacute Bacterial Endocarditis (SBE) after dental procedures
• Enter bloodstream during dental work → colonize damaged heart valves

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STEP 6: ENTEROCOCCUS - γ-hemolytic (Group D)

Key Properties:

• Grow in 6.5% NaCl (salt-tolerant - distinguishes from Strep)
• Grow at 10-45°C and at high pH
• Hydrolyze esculin in bile (esculin test +ve)
• PYR test positive
• Normal GI flora

Clinical Diseases:

THE KEY ISSUE - Antibiotic Resistance:

E. faecium is more likely to be VRE than E. faecalis. VRE = major nosocomial problem in ICUs.

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QUICK COMPARISON TABLE (High Yield)

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⚡ LAST-MINUTE MNEMONICS

• ASO titre rises in GAS pharyngitis but NOT in skin infection (DNase-B rises in skin)
• RF after pharyngitis (not skin) - "throat to heart"
• PSGN after skin OR throat
• Penicillin does NOT prevent PSGN but DOES prevent RF
• Erysipelas = ONLY GAS (raised borders, upper dermis)
• Necrotizing fasciitis = GAS Type 1 (flesh-eating, surgical emergency)
• VRE = E. faecium > faecalis

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🛡️ IMMUNOLOGY - COMPLETE EXAM GUIDE

All 15 Weeks | Exam-Ready | Based on Abbas & Lichtman

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PART 1: THE FOUNDATIONS (Weeks 1-2)

W1 - Immune Receptors & Signal Transduction

The Two Arms of Immunity

Key Receptors to Know:

• Recognizes peptide + MHC (cannot recognize free antigen alone)
• Linked to CD3 complex for signal transduction
• Signal cascade: TCR → CD3 → ZAP-70 → PLC-γ → IP3 + DAG → Ca²⁺ + PKC → NFAT + NFκB → gene transcription → IL-2 production

• Surface IgM/IgD
• Recognizes free antigen (no MHC needed)
• Linked to Igα/Igβ (CD79a/b) for signaling

Key signal molecule: IL-2 = the master T cell growth factor. Produced by activated T cells → drives proliferation (autocrine).

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W2 - Lymphocyte Development & Antigen Receptor Gene Rearrangement

T Cell Development (in Thymus)

Bone Marrow stem cell → Thymus
↓
Double Negative (DN): CD4⁻ CD8⁻ → TCR gene rearrangement (VDJ)
↓
Double Positive (DP): CD4⁺ CD8⁺
↓
POSITIVE SELECTION (survive if TCR recognizes self-MHC)
↓
NEGATIVE SELECTION (die if TCR binds self-MHC too strongly → prevents autoimmunity)
↓
Single Positive: CD4⁺ OR CD8⁺ → exit to periphery

B Cell Development (in Bone Marrow)

Stem cell → Pro-B cell → Pre-B cell → Immature B cell → Mature B cell (IgM+IgD)
                              ↓
                    VDJ recombination of heavy chain
                    VJ recombination of light chain

MHC (Major Histocompatibility Complex) - EXTREMELY HIGH YIELD

Mnemonic: "8 × CD8 = Class I" (8 aa peptide, CD8+ T cell) "4 × CD4 = Class II" (CD4+ T cell, exogenous antigen)

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PART 2: T CELL IMMUNITY (Weeks 3-5)

W3 - Activation of T Lymphocytes

The Two-Signal Rule (CRITICAL)

Sequence of T Cell Activation:

1. DC captures antigen in tissues → migrates to lymph node
2. Naive T cell in lymph node recognizes peptide-MHC on DC (Signal 1)
3. CD28-B7 interaction (Signal 2)
4. T cell produces IL-2 → autocrine proliferation
5. Clonal expansion → thousands of daughter cells
6. Differentiation into effector T cells + memory T cells

Key Co-stimulatory & Inhibitory Molecules:

CTLA-4 and PD-1 are immune checkpoints - cancer immunotherapy (ipilimumab, pembrolizumab) blocks these to unleash T cells against tumors.

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W4 - CD4⁺ Effector T Cells (T Helper Cells)

Mnemonic for Th1 vs Th2:

• Th1 = "Think intracellular" → IFN-γ → macrophage activation
• Th2 = "Two allergies" → IL-4/5 → IgE/eosinophils → worms + allergy

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W5 - CD8⁺ Effector T Cells (Cytotoxic T Lymphocytes - CTLs)

• Recognize peptide + MHC Class I (on ALL nucleated cells)
• Kill virally infected cells and tumor cells

Killing Mechanisms:

Key: CTLs kill by apoptosis, not necrosis. They do NOT use phagocytosis.

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PART 3: B CELL & ANTIBODY IMMUNITY (Weeks 6-7)

W6 - B Cell Activation & Antibody Production

Two Types of B Cell Responses:

• Antigen: Protein antigens
• Requires CD4+ T helper cell (Tfh)
• CD40L (T cell) + CD40 (B cell) = essential interaction
• Results in: Class switching, affinity maturation, memory B cells, plasma cells

• Antigen: Polysaccharides, lipopolysaccharides (no protein)
• No T cell help needed
• Only IgM produced (no class switching)
• No memory cells (reason why polysaccharide vaccines are weak in infants)

Germinal Center Reaction (key events):

1. Activated B cell enters germinal center in lymph node
2. Somatic hypermutation → mutate Ig variable regions → higher affinity antibodies
3. Affinity maturation → B cells with best antigen-binding selected (others die by apoptosis)
4. Class switch recombination → IgM → IgG, IgA, or IgE
5. Output: Plasma cells (antibody factories) + Memory B cells

Antibody Classes - HIGH YIELD:

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W7 - Effector Mechanisms of Humoral Immunity

Complement Pathways:

• C3b = opsonin (coats bacteria for phagocytosis)
• C3a, C5a = anaphylatoxins (mast cell degranulation, inflammation)
• C5a = most potent chemotactic factor
• MAC (C5b-9) = membrane attack complex → lysis

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W8 - Specialized Immunity at Epithelial Barriers

• IgA is the key antibody at mucosal surfaces
• Secretory IgA = dimer linked by J chain + secretory component (protects from proteases)
• Sites: gut (GALT), respiratory tract (BALT), genitourinary tract

• Eye (anterior chamber)
• Brain (blood-brain barrier; CNS)
• Testis
• Placenta/fetus
• Mechanisms: TGF-β, IL-10, FasL expression, regulatory T cells

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PART 4: REGULATION & SPECIAL TOPICS (Weeks 9-12)

W9 - Immunologic Tolerance & Autoimmunity

Tolerance = Unresponsiveness to self-antigens

• Negative selection kills self-reactive T cells
• AIRE (Autoimmune Regulator) gene in thymus expresses peripheral tissue antigens → allows deletion of self-reactive T cells
• AIRE mutations → APS-1 (Autoimmune Polyglandular Syndrome type 1)

Autoimmunity - When tolerance breaks:

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W10 - Immunity to Microbes

• IFN-α/β (Type I interferons) = produced by virus-infected cells → antiviral state in neighbors
• IFN-γ (Type II interferon) = produced by Th1, NK cells → activates macrophages (kills intracellular bacteria)
• TNF = macrophage product → fever, inflammation, cachexia (wasting in TB)

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W11 - Transplantation Immunology

Types of Grafts:

HLA Matching:

• HLA-A, HLA-B, HLA-C (Class I) + HLA-DR, HLA-DP, HLA-DQ (Class II)
• HLA-DR mismatch = strongest predictor of rejection
• Closer the match → less rejection

Rejection Types - HIGH YIELD:

Immunosuppression:

• Cyclosporine / Tacrolimus → block calcineurin → no IL-2 → no T cell activation
• Mycophenolate → blocks purine synthesis → no lymphocyte proliferation
• Steroids → broad anti-inflammatory
• Anti-thymocyte globulin → depletes T cells

Graft-versus-Host Disease (GVHD):

• Occurs in bone marrow transplant (donor T cells attack recipient)
• Acute GVHD: skin rash, diarrhea, liver failure
• Chronic GVHD: resembles autoimmune disease
• GVT (Graft-vs-Tumor) effect = donor T cells also kill residual tumor cells (beneficial!)

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W12 - Immunity to Tumors

Tumor Antigens:

Anti-Tumor Immunity:

• CD8+ CTLs = primary killers of tumor cells
• NK cells = kill tumor cells that downregulate MHC Class I (virus trick → NK catches it)
• Macrophages (M1) = kill tumors

Immune Evasion by Tumors:

• Downregulate MHC Class I → hide from CTLs
• Express PD-L1 → exhausts T cells
• Secrete TGF-β, IL-10 → immunosuppressive microenvironment
• Recruit Tregs into tumor

Cancer Immunotherapy:

• Checkpoint inhibitors: Anti-PD-1 (pembrolizumab), Anti-CTLA-4 (ipilimumab) → unleash T cells
• CAR-T cells: Patient's T cells engineered to target tumor antigens
• Monoclonal antibodies: Rituximab (anti-CD20, B cell lymphoma), Trastuzumab (anti-HER2)

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PART 5: HYPERSENSITIVITY & IMMUNODEFICIENCY (Weeks 13-15)

W13-W14 - Hypersensitivity (Coombs & Gell Classification) - MOST HIGH YIELD

The 4 Types - Master This Table:

Type I Deep Dive:

• Then: antihistamines, corticosteroids, bronchodilators

Type IV Subtypes:

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W14 - Allergy (Type I Hypersensitivity in Detail)

Asthma:

• Early phase (mins): bronchoconstriction from histamine + leukotrienes
• Late phase (hours): eosinophil infiltration → chronic inflammation
• Treatment: β2 agonists (bronchodilation), steroids (anti-inflammatory), leukotriene antagonists (montelukast)

Desensitization (Allergy shots):

• Repeated low-dose antigen → induces IgG ("blocking antibody") instead of IgE
• Promotes Treg activity
• Shifts Th2 → Th1

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W15 - Immunodeficiencies - HIGH YIELD

Primary (Congenital) Immunodeficiencies:

Secondary (Acquired) Immunodeficiencies:

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⚡ MASTER SUMMARY - QUICK RECALL TABLES

CD Markers - Must Know:

Cytokines - Must Know:

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Final Mnemonics:

• Anaphylactic (Type I - IgE)
• Cytotoxic (Type II - IgG/IgM)
• Immune complex (Type III)
• Delayed (Type IV - T cells)

• 0-6 months → SCID (no T/B cells)
• 6-12 months → Bruton's (maternal IgG wanes at 6 months)
• Childhood → DiGeorge, CGD, Wiskott-Aldrich
• Adulthood → CVID, Selective IgA deficiency

• C1q/C2/C4 deficiency → SLE-like disease
• C3 deficiency → recurrent severe bacterial infections
• C5-C9 deficiency → Neisseria infections only
• C1-inh (C1 inhibitor) deficiency → Hereditary Angioedema

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1. Hypersensitivity Types I-IV (with examples)
2. MHC Class I vs II
3. T cell vs B cell activation
4. Primary immunodeficiencies (Bruton's, DiGeorge, SCID especially)
5. Complement pathways and deficiencies

🛡️ IMMUNOLOGY - PART 1

Immune Receptors, MHC & Lymphocyte Development (Weeks 1-2)

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THE BIG PICTURE FIRST

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WEEK 1 - Immune Receptors & Signal Transduction

Innate Receptors (PRRs)

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TCR Signal Transduction (T Cell Receptor)

TCR recognizes peptide-MHC
        ↓
CD3 complex (ζ, γ, δ, ε chains) → signal transduction
        ↓
ZAP-70 (kinase) activated
        ↓
PLC-γ activated → cleaves PIP2 into:
   IP3 → Ca²⁺ release → NFAT (transcription factor)
   DAG → PKC → NFκB (transcription factor)
        ↓
NFAT + NFκB → nucleus → IL-2 gene transcription
        ↓
IL-2 produced → T cell proliferates (autocrine)

IL-2 = master T cell growth factor. This is what Cyclosporine/Tacrolimus block (by blocking calcineurin → no NFAT → no IL-2).

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BCR Signal Transduction (B Cell Receptor)

• Surface BCR = IgM + IgD
• Signaling via Igα/Igβ (CD79a/CD79b)
• B cell gets activated → proliferates → becomes plasma cell (antibody factory)

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WEEK 2 - Lymphocyte Development & Gene Rearrangement

T Cell Development (Thymus) - The Journey

Bone Marrow Stem Cell
        ↓ (migrate to thymus)
Double Negative (DN): CD4⁻ CD8⁻
→ TCR gene rearranged here (VDJ for β chain, VJ for α chain)
→ RAG1/RAG2 enzymes do the cutting
        ↓
Double Positive (DP): CD4⁺ CD8⁺
        ↓
✅ POSITIVE SELECTION (in thymic cortex)
→ Survive ONLY if TCR can weakly bind self-MHC
→ Those that can't bind self-MHC → die by neglect
→ Ensures T cells are "MHC-restricted"
        ↓
❌ NEGATIVE SELECTION (in thymic medulla)
→ Die if TCR binds self-MHC TOO strongly
→ Prevents autoimmunity
→ AIRE gene presents tissue antigens in thymus for this selection
        ↓
Single Positive: CD4⁺ OR CD8⁺
→ Exit thymus as naive T cells

AIRE mutation → APS-1 (Autoimmune Polyglandular Syndrome) - self-reactive T cells escape, attack multiple organs.

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B Cell Development (Bone Marrow)

Stem cell → Pro-B → Pre-B → Immature B → Mature B cell (IgM + IgD on surface)
              ↓          ↓
         Heavy chain   Light chain
         VDJ recomb    VJ recomb

• Immature B cells that bind strongly to self-antigen in bone marrow → clonal deletion or receptor editing (rearrange new light chain)
• If still self-reactive → anergy (functional silence)

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MHC (Major Histocompatibility Complex) - THE MOST IMPORTANT TABLE IN IMMUNOLOGY

Memory trick: "CD8 × 8 = Class I" (8-9 aa peptide, CD8+ T cell) "CD4 × 4 = Class II" (CD4+ T cell, exogenous antigen)

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APCs - Antigen Presenting Cells

• Immature DC in tissues: high phagocytosis, low T cell activation
• After antigen capture → matures → migrates to lymph node
• Mature DC: low phagocytosis, HIGH T cell activation (upregulates B7, MHC II, CCR7)

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V(D)J Recombination - Source of Antibody & TCR Diversity

• V = Variable, D = Diversity, J = Joining segments
• RAG1 + RAG2 = cut DNA at RSS (Recombination Signal Sequences)
• TdT (Terminal deoxynucleotidyl transferase) = adds random nucleotides at junctions → more diversity
• Result: Millions of different TCR/BCR specificities generated BEFORE ever seeing antigen

1. V(D)J recombination (combinatorial)
2. Junctional diversity (TdT adds N-nucleotides)
3. Multiple V, D, J segments
4. Heavy + Light chain pairing
5. Somatic hypermutation (after antigen exposure, in germinal center → affinity maturation)

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Quick Recall Summary - Part 1

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🛡️ IMMUNOLOGY - Taught Like You're Hearing It For The First Time

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What IS the Immune System?

• The skin and mucosa = border walls
• The immune system = the army that defends the country from invaders (bacteria, viruses, parasites)

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Division 1 - The INNATE Army 🪖

(Fast but dumb)

• Responds in minutes to hours
• Doesn't care WHICH enemy it is - just attacks anything foreign
• Like a security guard who stops everyone who looks suspicious
• NO memory - fights the same way every single time

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Division 2 - The ADAPTIVE Army 🎯

(Slow but smart)

• Takes days to weeks to respond (first time)
• Recognizes the specific enemy
• Has MEMORY - next time same enemy comes, responds FASTER and STRONGER (that's why vaccines work!)

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The Enemy Recognition Problem

Innate army uses: PRRs (Pattern Recognition Receptors)

• TLR4 - detects LPS (the coat of Gram-negative bacteria) → triggers alarm
• TLR3 - detects viral RNA → triggers antiviral response

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The Adaptive Army's ID Cards - TCR and BCR

• T cells have a TCR (T Cell Receptor)
• B cells have a BCR (B Cell Receptor = surface antibody)

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WEEK 1: How Does the TCR Signal Get Inside the Cell?

• TCR = a doorbell button (detects the visitor)
• CD3 complex = the wire connecting it to the alarm inside
• The alarm going off = cell activation

TCR sees enemy
    ↓
CD3 complex fires
    ↓
ZAP-70 (a switch) turns ON
    ↓
Two messengers created:
  → IP3 → releases Calcium → activates NFAT
  → DAG → activates PKC → activates NFκB
    ↓
NFAT + NFκB go into nucleus
    ↓
"MAKE IL-2!" command
    ↓
IL-2 produced → T cell divides and multiplies 💥

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BCR is Simpler

B cell = can grab antigen floating in blood T cell = needs antigen served on a specific plate (MHC)

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WEEK 2: How Are T and B Cells Made?

T Cell School - The THYMUS

• If YES → you pass, continue training
• If NO → you die (useless - can never recognize antigen)

• If YES → you die (dangerous - would cause autoimmune disease)
• If NO → you pass and graduate!

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B Cell School - Bone Marrow

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The MOST Important Concept: MHC (The Antigen Plate)

🔵 MHC Class I - "Internal Problems Tray"

• Found on EVERY cell in your body (every nucleated cell)
• Carries pieces of proteins made INSIDE the cell
• Normal cells show normal self-proteins → T cells ignore them
• Virus-infected cells show viral proteins → T cells recognize it as danger → KILL that cell
• Shown to CD8+ T cells (the killers)

Analogy: Every shop in a city is required to put its inventory list (what's being made inside) in the window. Police (CD8+ T cells) walk past - if they see something illegal (viral proteins) → they destroy the shop.

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🔴 MHC Class II - "External Threat Tray"

• Found ONLY on special cells called APCs (Dendritic cells, Macrophages, B cells)
• Carries pieces of proteins eaten from outside (phagocytosed bacteria)
• Shown to CD4+ T cells (the commanders)

Analogy: A spy (Dendritic cell) captures an enemy soldier, chops them up, and shows the badge (MHC II) to the general (CD4+ T cell). The general then commands the rest of the army.

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Simple Comparison Table:

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How is Diversity Created? (V(D)J Recombination)

• Many V segments (like suit: hearts, diamonds...)
• Many D segments (like number: 1, 2, 3...)
• Many J segments (like color: red, black...)

Analogy: Like a slot machine 🎰 - three random wheels spin independently and whatever combination comes up = that cell's unique receptor.

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Part 1 Recap - The 5 Things to Remember

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🛡️ IMMUNOLOGY - PART 2

T Cell Activation & CD4+ T Cell Subtypes

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First - Where Does Activation Happen?

1. Dendritic cells in your finger tissue grab pieces of the bacteria
2. They travel to the nearest lymph node (like reporting to HQ)
3. In the lymph node, millions of T cells are constantly circulating
4. The right T cell (whose key fits) meets the dendritic cell
5. ACTIVATION happens here

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The 2-Signal Rule - MOST IMPORTANT CONCEPT IN T CELL ACTIVATION

Analogy: Launching a nuclear missile needs TWO keys turned simultaneously. One person alone can't launch it. Both keys must turn at the same time.

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Signal 1 - The Specific Recognition

• TCR on T cell recognizes peptide + MHC on the Dendritic cell
• This is antigen-specific
• Alone → T cell becomes ANERGIC (switched off, useless, permanently silent)

Signal 2 - The Co-stimulation (The "OK it's real danger" signal)

• CD28 (on T cell) binds B7 / CD80 / CD86 (on Dendritic cell)
• This says: "Yes, this is a real infection. GO!"
• Only activated Dendritic cells express B7 (not normal resting cells)

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What happens with and without Signal 2:

This is why tumors are dangerous - they show antigen (Signal 1) but no B7 (no Signal 2) → T cell becomes anergic → tumor escapes.

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What Happens After Activation?

Signal 1 + Signal 2 received
        ↓
T cell makes IL-2 (the "multiply" signal)
        ↓
IL-2 acts on SAME T cell (autocrine) → massive proliferation
        ↓
CLONAL EXPANSION - one T cell becomes thousands
        ↓
Differentiation into:
   → EFFECTOR T cells (go fight NOW)
   → MEMORY T cells (remember for next time)

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The Brakes - Inhibitory Molecules

Cancer trick: Tumor cells put PD-L1 on their surface → they plug into the T cell's PD-1 → T cell shuts down → tumor survives.

Cancer treatment (checkpoint inhibitors): Drugs like Pembrolizumab (anti-PD-1) and Ipilimumab (anti-CTLA-4) BLOCK these brakes → T cells stay active → kill tumor. 💊

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CD4+ T Cells - The Commanders

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The 4 Main CD4+ Subtypes:

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🔴 Th1 - The "Intracellular Hunter"

Th1 releases IFN-γ
        ↓
IFN-γ hits Macrophage
        ↓
Macrophage ACTIVATES (becomes a killing machine)
        ↓
Kills intracellular bacteria

Analogy: Th1 is like a special operations commander who calls in airstrikes (IFN-γ) on specific targets that regular soldiers can't reach.

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🔵 Th2 - The "Allergy/Parasite Commander"

• IL-4 → tells B cells to make IgE (the allergy antibody)
• IL-5 → activates eosinophils (eosinophils attack worms)
• IL-13 → causes mucus production, airway hyperreactivity

Analogy: Th2 is like a commander who calls in specialized units (IgE for mast cells, eosinophils for worms). But when overactivated with no worms around → causes asthma, hay fever, food allergies.

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🟡 Th17 - The "Fungus & Extracellular Bacteria Fighter"

• IL-17 → recruits neutrophils to the site
• IL-22 → strengthens epithelial barrier

Analogy: Th17 calls in the riot police (neutrophils) to flush out bacteria from surfaces.

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🟢 Treg - The "Peacekeeper"

• IL-10 + TGF-β → calms down other T cells and macrophages
• Keeps immune responses from going too far

Analogy: Tregs are the UN peacekeepers that stop the fighting once the enemy is defeated. Without them → the army keeps fighting and destroys friendly territory (= autoimmune disease).

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Quick Visual Summary of CD4+ Subtypes:

                    CD4+ T cell activated
                           ↓
    What cytokines are in the environment?
          ↙        ↓        ↓        ↘
       IL-12    IL-4    IL-6+TGF-β   TGF-β alone
         ↓        ↓        ↓              ↓
       Th1      Th2      Th17           Treg
         ↓        ↓        ↓              ↓
       IFN-γ   IL-4,5   IL-17,22      IL-10,TGF-β
         ↓        ↓        ↓              ↓
  Kills intra-  IgE +   Neutrophil    Suppresses
  cellular     Eosin.   recruitment   immune response
   bacteria    Worms     Fungi/GNB     Tolerance

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CD8+ T Cells - The Killers (Week 5)

How They Kill:

CD8+ T cell docks onto infected cell
        ↓
Releases PERFORIN → punches holes in target cell membrane
        ↓
GRANZYMES rush in through the holes
        ↓
Granzymes activate CASPASES inside cell
        ↓
APOPTOSIS - cell dies in a controlled way (no explosion, no inflammation)

CD8+ T cell expresses FasL (Fas Ligand)
        ↓
Binds Fas receptor on target cell
        ↓
Apoptosis triggered

Key point: CD8+ T cells kill by APOPTOSIS (clean, controlled death). NOT by eating the cell. NOT by explosion (necrosis).

Why apoptosis? So the virus inside the cell gets destroyed WITHOUT releasing it into the surroundings. Smart!

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Quick Comparison: CD4+ vs CD8+

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Part 2 - 5 Things to Remember

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🛡️ IMMUNOLOGY - PART 3

B Cells, Antibodies & Complement

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What Are B Cells?

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How Does a B Cell Get Activated?

Step 1 - B Cell Sees Antigen

• B cell's receptor (BCR = surface IgM) grabs antigen floating in blood/lymph
• B cell chops up the antigen and presents it on MHC Class II

Step 2 - T Cell Comes to Help

• A CD4+ Tfh cell (T follicular helper - a special Th subtype) recognizes the peptide on MHC II
• CD40L (on T cell) locks onto CD40 (on B cell) → "YOU HAVE PERMISSION TO FIRE"
• T cell also releases cytokines (IL-4, IL-21) to further boost B cell

Step 3 - B Cell Activates

• B cell proliferates
• Enters the Germinal Center (a special zone inside lymph node)
• Magic happens there...

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The Germinal Center - Where Antibodies Get UPGRADED

1. Somatic Hypermutation 🔬

• The B cell's antibody gene randomly mutates
• Creates B cells with slightly different antibodies
• Some mutations make the antibody bind the enemy better
• Some make it bind worse

2. Affinity Maturation 🏆

• B cells that make better-fitting antibodies → survive and multiply
• B cells that make worse antibodies → die (apoptosis)
• Result: Over time, antibodies get stronger and stronger

Analogy: Like a sporting tryout. Each round, only the best players survive. After several rounds, you have an elite team.

3. Class Switch Recombination 🔄

• B cell starts making IgM (basic antibody)
• Gets instructions from T cell cytokines to switch to a better antibody class
• IL-4 → switch to IgE or IgG
• TGF-β → switch to IgA
• Same antigen specificity, different antibody class

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The 5 Antibody Classes - LEARN THESE COLD

🥇 IgG - The Main Fighter

• Most abundant antibody in blood
• Only antibody that crosses the placenta → gives baby protection for first 6 months
• Opsonization (coats bacteria so macrophages can eat them)
• Neutralization (blocks viruses from entering cells)
• Activates complement
• 4 subclasses: IgG1, IgG2, IgG3, IgG4

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🥈 IgM - The First Responder

• First antibody produced in any infection
• Pentamer (5 Y-shapes joined together) → very large
• Best complement activator (classical pathway)
• Does NOT cross placenta
• Found on surface of naive B cells (IgM + IgD together)

Exam tip: If a question says "first antibody in infection" = IgM. Rising IgM = CURRENT infection.

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🥉 IgA - The Mucosal Guardian

• Found in secretions: saliva, tears, breast milk, gut, respiratory tract
• Dimer in secretions (two Y-shapes joined)
• Protected by a secretory component (prevents digestion by enzymes)
• First line of defense at mucosal surfaces
• Passed from mother to baby in breast milk (passive immunity)

Analogy: IgA is the border patrol stationed at all entry points (mouth, nose, gut).

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IgE - The Allergy Antibody

• Lowest concentration in blood normally
• Binds to mast cells and basophils via Fc receptor
• When antigen cross-links IgE on mast cell → DEGRANULATION → histamine → allergic reaction
• Also important for anti-parasitic (helminth) immunity

High IgE = think: allergy, asthma, parasites, Hyper-IgE syndrome (Job syndrome)

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IgD - The Mystery One

• Found on surface of naive B cells alongside IgM
• Function not fully understood
• Not a major exam focus - just know it's a B cell surface marker

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Quick Antibody Summary Table:

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How Do Antibodies Actually Fight?

1. Neutralization 🛡️

• Antibody physically blocks the virus/toxin from binding to your cell
• Like putting a cap on a key so it can't open the lock

2. Opsonization 🎯

• IgG coats the bacteria
• Macrophages and neutrophils have Fc receptors - they grab the IgG tail
• Phagocytose and destroy the bacteria
• Like putting a "EAT ME" sticker on bacteria

3. Complement Activation 💣

• IgM or IgG on bacteria → activates complement → bacteria gets blown up

4. ADCC (Antibody-Dependent Cell Cytotoxicity)

• IgG coats virus-infected cell
• NK cells grab the IgG tail via their CD16 receptor
• NK cell kills the infected cell

5. Mast Cell Degranulation

• IgE on mast cells → antigen cross-links → histamine release → allergy

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COMPLEMENT - The Chemical Bomb System

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3 Ways to Start the Cascade (3 Pathways):

Mnemonic: "CALM" = Classical-Antibody, Lectin-Mannose, Alternative-spontaneous

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All 3 pathways converge at C3:

Classical ──┐
Lectin ─────┼──→ C3 convertase → C3 splits into C3a + C3b
Alternative ┘
                    ↓                    ↓
                   C3a              C3b coats bacteria
              (anaphylatoxin)        (opsonin = "eat me" tag)
                    ↓
              Mast cell degranulation
              (inflammation)
                    ↓
              C5 → C5a + C5b
                    ↓        ↓
              C5a =      C5b + C6+C7+C8+C9
         most powerful         ↓
         chemotactic      MAC (Membrane
           factor       Attack Complex)
                              ↓
                    Punches holes in bacteria
                    → bacteria bursts and dies

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Key Complement Products - HIGH YIELD:

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Complement Deficiency Diseases:

Exam favourite: Patient with recurrent Neisseria meningitidis = C5-C9 deficiency

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T-Dependent vs T-Independent Antigens

This is why polysaccharide vaccines (like PPSV23) don't work well in infants under 2 - their T-independent response is weak. That's why we use conjugate vaccines (polysaccharide + protein carrier = now T-dependent) like PCV13.

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Part 3 Summary - 5 Things to Remember

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🛡️ IMMUNOLOGY - PART 4

Tolerance, Autoimmunity, Transplant & Tumor Immunity

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SECTION 1: Immunologic Tolerance

What is Tolerance?

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How Does the Body Prevent Attacking Itself?

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Central Tolerance (in Thymus & Bone Marrow)

• Remember negative selection? T cells that bind self-antigens TOO strongly → killed
• Special gene called AIRE makes the thymus display proteins from other organs (thyroid, pancreas, eye)
• T cells that react to these → deleted before they leave

Analogy: Like a background check BEFORE someone joins the army. Anyone with criminal intent gets rejected at the gate.

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Peripheral Tolerance (in Tissues - Second Line of Defense)

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SECTION 2: Autoimmunity

Why Does Autoimmunity Happen?

1. Molecular mimicry - microbial antigen looks similar to self-antigen
   • Example: GAS M protein looks like cardiac myosin → Rheumatic fever
   • Example: Campylobacter antigen looks like nerve myelin → Guillain-Barré syndrome
2. Bystander activation - tissue damage releases hidden self-antigens → immune system sees them for first time
3. AIRE deficiency - self-reactive T cells escape thymus
4. Loss of Tregs (FOXP3 mutation) → no suppression → IPEX syndrome

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Major Autoimmune Diseases - HIGH YIELD TABLE:

Mnemonic for SLE antibodies: "DNA So Many" = dsDNA, Sm (Smith antigen)

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SECTION 3: Transplantation Immunology

Why Does the Body Reject Transplants?

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Types of Grafts:

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HLA Matching

• HLA-DR (Class II) - mismatch here = worst rejection
• HLA-A, HLA-B (Class I)

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Types of Rejection - EXTREMELY HIGH YIELD:

Hyperacute = antibody already made BEFORE transplant (like if patient had previous transplant or pregnancy)

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Immunosuppression - How We Prevent Rejection:

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Graft vs Host Disease (GVHD) - Unique to Bone Marrow Transplant

Happens in bone marrow transplant because you're transplanting IMMUNE CELLS (T cells) into the patient.

• Skin rash
• Diarrhea
• Liver dysfunction (jaundice)

• Looks like autoimmune disease
• Dry eyes, dry mouth, skin fibrosis, joint stiffness

• Donor T cells also attack remaining cancer cells in the host
• This is actually therapeutic - helps cure leukemia

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SECTION 4: Tumor Immunology

Does the Immune System Fight Cancer?

• Immunocompromised patients (HIV, transplant patients on immunosuppressants) have MUCH higher cancer rates
• Cancers have immune cell infiltrates (immune cells surrounding them)

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Tumor Antigens (What the Immune System Recognizes):

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How the Immune System Fights Cancer:

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How Cancer Escapes the Immune System:

This is what makes cancer so dangerous.

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Cancer Immunotherapy - The Revolution:

How they work: Remove the brakes → T cells stay active → kill tumor cells Side effect: Autoimmune reactions (immune system over-activated)

• Take patient's own T cells
• Engineer them in lab to target specific tumor antigen (e.g. CD19 on B cell lymphoma)
• Infuse back → super-powered T cells hunt and kill tumor

• Rituximab → Anti-CD20 → kills B cell lymphomas
• Trastuzumab (Herceptin) → Anti-HER2 → breast cancer
• Cetuximab → Anti-EGFR → colorectal cancer

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Part 4 Summary - 5 Things to Remember

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🛡️ IMMUNOLOGY - PART 5 (FINAL)

Hypersensitivity + Immunodeficiencies

⚠️ Most Exam-Heavy Topics of All

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HYPERSENSITIVITY

What is Hypersensitivity?

Analogy: An overzealous security guard who not only stops the thief but also smashes the entire shop.

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TYPE I - Immediate Hypersensitivity (Allergy)

⚡ Seconds to Minutes

The Simple Story:

Allergen enters body (pollen, peanut, bee sting)
        ↓
Th2 cells activated → IL-4 produced
        ↓
B cells switch to IgE
        ↓
IgE binds to Fc receptors on MAST CELLS and BASOPHILS
        ↓
Person is now "sensitized" - nothing happens yet

Same allergen enters again
        ↓
Allergen cross-links IgE on mast cell surface
        ↓
DEGRANULATION - mast cell explodes its granules
        ↓
Histamine + Leukotrienes + Prostaglandins released
        ↓
Vasodilation + Bronchoconstriction + Itching + Swelling

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Mediators Released:

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Clinical Examples:

• Anaphylaxis - systemic, life-threatening (bee sting, peanut, penicillin)
• Asthma - bronchospasm
• Allergic rhinitis - hay fever
• Urticaria - hives
• Atopic dermatitis - eczema
• Food allergy

Treatment of Anaphylaxis:

1. Epinephrine IM (FIRST AND MOST IMPORTANT - reverses everything)
2. Antihistamines (H1 blockers)
3. Corticosteroids
4. Bronchodilators (salbutamol)

Exam tip: Anaphylaxis treatment = EPINEPHRINE FIRST. Always.

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TYPE II - Cytotoxic Hypersensitivity

⏱️ Hours

The Simple Story:

1. Complement activation → MAC punches holes → cell lyses
2. Phagocytosis → IgG coats cell → macrophages eat it (opsonization gone wrong)
3. ADCC → NK cells kill IgG-coated cells

• Instead of destroying the cell, antibody changes its function
• Example: Ab STIMULATES receptor → overactivation
• Example: Ab BLOCKS receptor → no function

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Clinical Examples:

Trick to remember Graves vs Myasthenia:

• Graves = Gas pedal stuck (stimulating = hyperthyroid, overactive)
• Myasthenia = Muscle paralysis (blocking = no signal = weak muscles)

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TYPE III - Immune Complex Hypersensitivity

⏱️ 6-12 Hours

The Simple Story:

Analogy: Imagine police (antibodies) chasing criminals (antigens) through a city. They crash into each other (immune complex) right in the middle of a hospital (kidney). The explosion damages the hospital, not just the criminal.

Antigen + Antibody → Immune Complex formed
        ↓
Complex deposits in tissues (kidney, joints, vessels)
        ↓
Complement activated → C3a, C5a produced
        ↓
Neutrophils recruited → release enzymes
        ↓
TISSUE DAMAGE (not the pathogen - the bystander!)

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Clinical Examples:

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TYPE IV - Delayed Hypersensitivity

⏱️ 48-72 HOURS (slowest of all)

The Simple Story:

1. Find the antigen-specific T cell
2. Activate it
3. T cell makes cytokines
4. Macrophages accumulate
5. THEN damage occurs

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The 4 Subtypes of Type IV:

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Clinical Examples:

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THE MASTER HYPERSENSITIVITY TABLE:

Mnemonic: "ACID"

• Anaphylactic (I)
• Cytotoxic (II)
• Immune complex (III)
• Delayed (IV)

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IMMUNODEFICIENCIES

Primary (Born With It) vs Secondary (Acquired Later)

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PRIMARY IMMUNODEFICIENCIES

Grouped by WHAT is missing:

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GROUP 1: No B Cells / No Antibodies

• Gene: BTK (Bruton's Tyrosine Kinase) mutation
• Only in boys (X-linked)
• B cells don't mature → NO B cells → NO antibodies
• Fine at birth (has maternal IgG via placenta)
• Gets sick at 6 months when maternal IgG disappears
• Infections: Encapsulated bacteria (S.pneumo, H.flu, Neisseria)
• Treatment: IV immunoglobulin (IVIG) every month

• Most common primary immunodeficiency overall
• Only IgA missing, others normal
• Usually asymptomatic (most people don't know they have it)
• Some get recurrent sinus/chest infections, Giardia
• Danger: Give blood transfusion → anaphylaxis (recipient's anti-IgA antibodies react)

• Low IgG, IgA, IgM
• B cells present but don't mature into plasma cells
• Presents in adults (20-30 years old)
• Recurrent sinopulmonary infections, autoimmunity, lymphoma risk

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GROUP 2: No T Cells

• Chromosome 22q11 deletion
• Thymus absent or small → no T cells
• Classic triad:
  • Cardiac defects (conotruncal - Tetralogy of Fallot, truncus arteriosus)
  • Abnormal facies
  • Thymic hypoplasia (no T cells)
  • Cleft palate
  • Hypocalcemia (parathyroid glands absent → tetany/seizures in newborn)
• Mnemonic: CATCH-22 (Cardiac, Abnormal face, Thymic, Cleft palate, Hypocalcaemia - chromosome 22)
• Infections: Viral and fungal (need T cells to fight these)

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GROUP 3: No T cells AND No B cells (SCID)

• The most severe of all - no T cells AND no B cells
• Most common cause: ADA deficiency (Adenosine Deaminase enzyme) → toxic metabolites kill lymphocytes
• Also: RAG1/RAG2 mutations → no V(D)J recombination
• Presents in first weeks-months of life
• Any infection = potentially fatal (viral, bacterial, fungal)
• Classic: "Bubble boy" (David Vetter - lived in sterile plastic bubble)
• Treatment: Bone marrow transplant (curative!)
• Danger: Do NOT give live vaccines → they will cause disease

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GROUP 4: Phagocyte Defects

• NADPH oxidase gene mutation
• Neutrophils can engulf bacteria but CANNOT kill them (no respiratory burst, no ROS)
• Infections specifically with catalase-positive organisms (these destroy their own H2O2 so neutrophil can't use it against them)
  • Staph aureus, Aspergillus, Serratia, Nocardia, Burkholderia
• Granulomas form because bacteria can't be killed → body walls them off
• Diagnosis: DHR test (Dihydrorhodamine) or NBT test (Nitroblue Tetrazolium) - no color change
• Treatment: Prophylactic trimethoprim-sulfamethoxazole + IFN-γ

• LYST gene mutation → lysosomes can't fuse properly
• Neutrophils have giant granules (visible on blood smear!)
• Partial albinism (melanocytes affected too)
• Recurrent pyogenic infections
• Nerve problems

• CD18 (integrin) mutation → neutrophils can't stick to vessel walls → can't get to infection site
• Hallmark: Delayed separation of umbilical cord (>3 weeks after birth)
• Infections without pus formation (no neutrophils arrive)
• Very high circulating neutrophil count (can't leave blood)

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GROUP 5: Complement Deficiencies

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Quick Summary Table - Primary Immunodeficiencies:

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SECONDARY IMMUNODEFICIENCIES (Acquired)

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PART 5 Summary - 6 Things to Remember

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🏁 ALL 5 PARTS OF IMMUNOLOGY COMPLETE!

• ✅ Part 1 - Innate vs Adaptive, Receptors, MHC, Lymphocyte development
• ✅ Part 2 - T cell activation (2-signal rule), CD4+ subtypes, CD8+ killing
• ✅ Part 3 - B cells, Antibody classes, Complement pathways
• ✅ Part 4 - Tolerance, Autoimmunity, Transplant, Tumor immunity
• ✅ Part 5 - Hypersensitivity Types I-IV + All immunodeficiencies