🧠 IMMUNOLOGY - A Complete Lecture-Style Explanation for Everyone

PAGE 1: IMMUNOLOGY - LYMPHOID STRUCTURES

What is the Immune System?

IMMUNE SYSTEM ORGANS

1° (Primary) Organs - "The Training Camps"

Think of Bone Marrow as the factory inside your bones. All blood cells - red blood cells, white blood cells, platelets - are born here. For the immune system specifically, something called B cells (a type of white blood cell responsible for making antibodies - weapons against infection) are both born AND trained here. "B" for Bone marrow - easy to remember!

The Thymus is a small gland located just behind your breastbone (sternum), in your chest. T cells are born in bone marrow but travel to the thymus to be trained/matured. "T" for Thymus. Think of thymus as a military academy - it takes raw recruits (immature T cells) and turns them into highly skilled soldiers. Importantly, the thymus shrinks (involutes) as you age - it is biggest in childhood and almost gone by adulthood.

2° (Secondary) Organs - "The Battlefields"

These are spread throughout your body strategically. Think of them as checkpoints or patrol stations. When a germ enters your body, it gets caught at one of these stations and the immune cells stationed there respond.

Antigen = any foreign substance that triggers an immune response. It could be a piece of a bacterium, a virus, pollen, etc. The secondary organs are places where immune cells physically encounter these antigens and begin the fight.

LYMPH NODE - "The Regional Police Station"

• Lymph node = small bean-shaped structures scattered throughout your body (you can feel them in your neck, armpits, groin when they are swollen)
• Many afferents = "afferent" means "coming in." Lymph nodes receive fluid (called lymph) from many vessels bringing in potential threats from different areas of the body. Think of many roads leading into a city
• 1 or more efferents = "efferent" means "going out." Fluid leaves via fewer vessels - like a funnel, many roads in, fewer roads out. This slows things down so immune cells have time to inspect everything
• Encapsulated = surrounded by a tough outer covering (capsule) - like a factory wall
• Trabeculae = internal walls/partitions inside the lymph node that divide it into compartments. Like walls inside a building

Macrophages = big eater cells ("macro" = big, "phage" = eater). They are like garbage collectors - they engulf and destroy anything suspicious without being picky. That is "nonspecific filtration" - no target needed, they eat everything foreign. B and T cells circulate through (patrol), and if they find their specific target, the immune response is activated.

FOLLICLE - "The B-Cell Neighborhood"

Cortex = outer layer of the lymph node. The follicle is a specific zone within the outer cortex. This is where B cells live and multiply.

Primary (1°) follicles = resting follicles. Dense means tightly packed with B cells. Quiescent = quiet, inactive, not currently fighting. These are B cells just waiting on standby - no infection is happening right now.

Secondary (2°) follicles = active follicles. When an infection happens, B cells in the follicle start multiplying rapidly, forming a germinal center - a light-colored (pale) central zone where B cells are rapidly dividing and improving their antibodies. This is like a training session happening in real-time during a war. When your lymph nodes are swollen and tender, germinal centers are forming - your B cells are actively fighting!

MEDULLA - "The Inner Core"

• Medullary cords = dense clusters of cells including lymphocytes (B and T cells) and plasma cells (B cells that have transformed into antibody factories - they do nothing but pump out antibodies all day)
• Medullary sinuses = open channels/passages filled with fluid. They contain reticular cells (structural support cells, like scaffolding) and macrophages (the eater cells)
• Think of the medulla as the inner industrial zone of the lymph node

The fluid in the sinuses eventually drains out of the lymph node through the exit vessels (efferent lymphatics), carrying filtered, cleaned lymph back into circulation.

PARACORTEX - "The T-Cell Zone"

The paracortex is a middle zone sandwiched between the follicle (outer, B-cell zone) and the medulla (inner zone). This is where T cells live.

High Endothelial Venules (HEV) = specialized blood vessels with tall, cube-shaped cells lining them. These are like special gates or revolving doors that allow T and B cells to move from the bloodstream into the lymph node. This is how immune cells "patrol" - they continuously enter and exit lymph nodes checking for threats.

DiGeorge syndrome = a genetic condition where the thymus doesn't develop properly. Without the thymus, T cells can't mature. No T cells = no paracortex activity. So in DiGeorge patients, the paracortex is underdeveloped because there are almost no T cells to populate it.

When a big infection happens (especially viral), T cells go into overdrive. The paracortex swells as T cells multiply massively - this is called paracortical hyperplasia (hyper = excess, plasia = growth). The entire lymph node gets bigger = lymphadenopathy (swollen lymph nodes). EBV (Epstein-Barr Virus) is the virus that causes mononucleosis ("mono" or "kissing disease") - a classic example where lymph nodes swell enormously.

PAGE 2: LYMPHATIC DRAINAGE ASSOCIATIONS

The Table Explained

Nodes under your jaw drain your mouth. If you have a mouth cancer, check under the jaw for swollen nodes.

Deep nodes in the neck drain the whole head and throat area. Swollen deep cervical nodes = think about throat infection, mono (EBV), Kawasaki disease, or cancer of the head/neck. Kawasaki disease = a childhood illness causing fever, rash, and inflammation of blood vessels.

Nodes just above the collarbone. This is clinically very important:

• Right supraclavicular node swelling = problem in the right chest (lungs, etc.)
• Left supraclavicular node (Virchow's node / Troisier's sign) = drains the ENTIRE abdomen and pelvis. A swollen left supraclavicular node is a RED FLAG for abdominal cancer (especially gastric/stomach cancer). Doctors check this routinely!

Mediastinum = the middle of the chest (between the lungs). Nodes here drain the airway and food pipe.

• Unilateral hilar node swelling = TB (tuberculosis)
• Bilateral hilar nodes = Sarcoidosis (an inflammatory disease)

"Hilar" = at the root of the lung (where blood vessels enter). Swollen hilar nodes = lung cancer or granulomatous disease (like sarcoidosis or TB).

Nodes in your armpit. They drain the entire arm and breast.

• Mastitis (breast infection) = swollen axillary nodes
• Breast cancer commonly spreads here first - surgeons remove and check axillary nodes in breast cancer surgery

Small nodes near the elbow. Classic finding in secondary syphilis (a sexually transmitted infection, stage 2). Swollen epitrochlear nodes + a generalized rash = think syphilis.

Deep abdominal nodes draining upper digestive organs.

"Mesenteric" = in the mesentery (the fold of tissue holding intestines). Associated with: mesenteric lymphadenitis (belly pain, especially in children mimicking appendicitis), Inflammatory Bowel Disease (Crohn's, Ulcerative colitis), Celiac disease.

Lower part of large intestine.

Sister Mary Joseph node = a swollen lymph node felt at the navel (belly button). It is a classic sign of intraabdominal malignancy, most commonly gastric (stomach) cancer. Named after a nun who was a surgical assistant who noticed this sign. It means cancer has spread to this node via lymphatics.

Nodes alongside the aorta (main artery in the abdomen). They drain the gonads (testes and ovaries). This is why testicular cancer and ovarian cancer spread to para-aortic nodes, NOT the inguinal nodes.

Pectinate line = a line inside the anal canal dividing upper from lower portions. Above it drains to internal iliac; below it drains to inguinal. Internal iliac nodes are important for sexually transmitted infections and lower urinary/rectal cancers.

Groin nodes. Everything external on the lower body and genitalia drains here.

Behind the knee. The mnemonic "pop-lateral" helps remember: popliteal drains the lateral aspect. Lateral foot/leg cellulitis (skin infection) → swollen popliteal node.

The Two Main Lymphatic Ducts

The body's lymph ultimately returns to the bloodstream. The right lymphatic duct collects lymph from only the right upper body (right arm, right side of chest and head) and empties into a vein in the right neck.

The thoracic duct is the main (bigger) duct - it drains everything ELSE (everything below the diaphragm + left side of chest and left arm). It empties on the LEFT side. Chylothorax = if the thoracic duct is injured (e.g., in chest surgery or trauma), milky white lymph leaks into the chest cavity - this milky fluid is called chyle (lymph mixed with fat). It is a serious complication.

PAGE 3: SPLEEN AND THYMUS

SPLEEN

LUQ = Left Upper Quadrant. When a doctor divides the abdomen into 4 quadrants, the top-left is LUQ. The spleen sits there, protected by lower ribs (9th to 11th). This is why a broken rib in the left lower chest can lacerate the spleen - a surgical emergency.

• Postsplenectomy = after spleen removal (surgery). Autosplenectomy = the spleen destroys itself (happens in sickle cell disease because repeated sickling damages the spleen over time)
• ↓ IgM = less IgM antibody produced. The spleen is the main place IgM is made against blood-borne antigens
• ↓ complement activation = complement is a system of proteins in blood that attacks bacteria. Less IgM = less complement gets turned on
• ↓ C3b opsonization = Opsonization means "marking for destruction." C3b is a complement protein that sticks to bacteria like a flag, marking them for macrophages to eat. Less C3b = fewer flags = bacteria escape detection
• ↑ susceptibility to encapsulated organisms = Encapsulated bacteria have a sugar coating (capsule) that makes them slippery and hard for immune cells to grab WITHOUT opsonization. The spleen is essential for handling these bacteria. Without a spleen, these bacteria can cause fatal infections

Mnemonic: "SHiNe" - After splenectomy, vaccinate against Streptococcus pneumoniae, Haemophilus influenzae type b, meNingococcus. These are the three most dangerous encapsulated bacteria for asplenic patients.

• Howell-Jolly bodies = small dark dots inside red blood cells. Normally the spleen removes them. Without a spleen, these nuclear remnants accumulate. Their presence on blood smear = marker of no spleen function
• Target cells = red blood cells that look like a target/bullseye under microscope. Also accumulate without a spleen
• Thrombocytosis = too many platelets in blood. Normally the spleen stores 1/3 of platelets. Without a spleen, all platelets circulate = thrombocytosis
• Lymphocytosis = too many lymphocytes in blood. The spleen sequesters (holds) some lymphocytes; without it, they all circulate freely

Spleen Zones:

The white pulp is the immune part of the spleen (like a mini lymph node embedded in the spleen). The PALS surrounds the central artery and is packed with T cells. Think: PALS = T cells hang around the artery

Just like lymph node follicles, the spleen has follicles in its white pulp where B cells live and can form germinal centers.

• Marginal zone = the border between white pulp (immune tissue) and red pulp (blood filtering tissue)
• APCs = Antigen-Presenting Cells = cells that eat foreign substances, chop them up, and display the pieces on their surface so T cells can recognize and respond to them. Think of them as detectives showing mug shots to T cells
• Since blood flows through the spleen, blood-borne antigens get captured here

THYMUS

Front-upper part of the chest, behind the breastbone. Children have a large thymus; adults barely have one.

This is the T cell school. Immature T cells (thymocytes) come in, get educated, and leave as competent T cells.

Pharyngeal pouches = embryological structures in the developing neck/throat area of a fetus.

• The epithelial lining (scaffold/architecture) of the thymus comes from the 3rd pharyngeal pouch (endoderm = inner layer of embryo)
• The lymphocytes (the actual T cells) come from mesoderm (middle layer of embryo - specifically from bone marrow precursors)
• Clinical pearl: DiGeorge syndrome = failure of 3rd pharyngeal pouch development → no thymus epithelium → no T cell education → immunodeficiency

• Cortex = outer layer of thymus = packed with immature, dark-staining thymocytes
• Medulla = inner layer = sparse, contains mature T cells
• Hassall corpuscles = small swirly structures unique to the thymus medulla. They are made of dead epithelial cells. Their function: support negative selection (removing dangerous T cells). You see these on histology of the thymus - they look like onion rings!

On a chest X-ray of a newborn, the thymus looks like a sail or a wave shape. This is normal. But by age 3, it starts shrinking. If you do NOT see a thymic shadow on a neonatal X-ray, that is abnormal and suggests DiGeorge or SCID.

Thymoma = a tumor of the thymus. It is associated with:

• Myasthenia gravis = an autoimmune disease causing muscle weakness (antibodies attack the connection between nerves and muscles)
• Superior vena cava syndrome = the tumor compresses the SVC (large vein), causing facial swelling and arm swelling
• Pure red cell aplasia = bone marrow stops making red blood cells
• Good syndrome = thymoma + low antibody levels (immunodeficiency)

PAGE 4: INNATE vs ADAPTIVE IMMUNITY

INNATE IMMUNITY - "The Rapid Response Team"

Neutrophils = first responders, arrive within minutes. They swarm the infection site and eat bacteria (like infantry soldiers). Macrophages = big eaters that also present antigens. Think of them as both soldiers AND detectives. Monocytes = macrophage precursors (monocytes live in blood; they move into tissues and become macrophages). Dendritic cells = the best antigen presenters. They pick up antigens, travel to lymph nodes, and show them to T cells - essentially messengers. Natural Killer (NK) cells = lymphoid origin but still part of innate immunity. They kill virus-infected cells and tumor cells WITHOUT needing to learn who the target is. Complement = a cascade of proteins in the blood that, when activated, can punch holes in bacteria. Physical epithelial barriers = your skin and mucous membranes are the actual first defense - just keeping things out. Secreted enzymes = like lysozyme in tears and saliva - they can break down bacterial walls.

The genes for innate immunity receptors are fixed - they are encoded directly in your DNA from birth, the same in every person. No modification needed. You are born with these defenses ready.

Nonspecific = it doesn't matter what the germ is - innate immunity responds the same way. Minutes to hours = incredibly fast. No memory = if the same germ attacks again, innate immunity responds exactly the same way - it does NOT remember past encounters.

• CRP (C-reactive protein) = a protein made by the liver during inflammation. Doctors check CRP blood levels to know if infection/inflammation is present
• Defensins = antimicrobial peptides - tiny proteins that poke holes in bacterial membranes
• Cytokines = chemical messengers between immune cells (like text messages saying "come here!" or "attack!" or "calm down")

Toll-like receptors (TLRs) = receptors on innate immune cells that act like smoke detectors - they are designed to recognize common patterns found on microbes.

• PAMPs (Pathogen-Associated Molecular Patterns) = patterns found on pathogens (foreign invaders). Examples: LPS (lipopolysaccharide, found on gram-negative bacteria's outer wall), Flagellin (the tail/propeller of bacteria), Nucleic acids (viral RNA or DNA).
• DAMPs (Damage-Associated Molecular Patterns) = patterns released when your own cells are damaged. Examples: mitochondrial DNA, histones (proteins that package DNA), heat shock proteins.
• When TLRs detect PAMPs or DAMPs, they activate NF-κB (a master switch transcription factor inside the cell nucleus), which turns on genes for pro-inflammatory cytokines - basically sounding the alarm throughout the body.

ADAPTIVE IMMUNITY - "The Special Forces"

T cells (cell-mediated), B cells (antibody-mediated), Circulating antibodies

V(D)J recombination = a molecular shuffling process. Think of it like shuffling a deck of cards - the genes that code for antibodies and T-cell receptors are shuffled in different combinations each time a lymphocyte is made. This creates an almost infinite variety of receptors, each one capable of recognizing a different antigen. This is how your body can fight viruses it has never seen before.

Highly specific = each T or B cell targets ONE specific antigen. Develops over long periods = takes days to weeks to ramp up (which is why you feel sick for a few days before getting better). Memory response is faster and more robust = the second time you see the same pathogen, the immune system responds much faster and stronger. This is the basis of vaccination - expose you to a harmless form of the pathogen so your immune system builds memory without getting sick.

Memory cells = activated B and T cells that persist long after infection is resolved. When the same antigen is encountered again, these memory cells respond far faster (hours instead of days). Immunosenescence = "immune aging." As you get older, adaptive immunity weakens. This is why older adults get sicker from infections and respond less well to vaccines.

IMMUNE PRIVILEGE

Normally the immune system attacks anything foreign. But some organs cannot afford inflammation because inflammation would destroy them:

• Eye = swelling/inflammation inside the eye would destroy vision
• Brain = inflammation (encephalitis) can cause massive damage
• Testes = sperm are formed after the immune system is already mature, so sperm antigens are "new" and could be attacked. Immune privilege protects them
• Placenta = the fetus is half "foreign" (father's genes) - immune privilege prevents the mother's immune system from attacking the fetus Allograft = a transplant from another person. These immune-privileged sites are better places for transplants (e.g., corneal transplants rarely need anti-rejection drugs).

PAGE 5: MHC (Major Histocompatibility Complex) I and II

What is MHC?

• HLA (Human Leukocyte Antigen) = the genes that code for MHC in humans. Every person has slightly different HLA genes (which is why organ transplants are difficult - the recipient's immune system sees the donor's HLA as foreign)
• TCR (T-Cell Receptor) = the receptor on T cells that reads the MHC. It is like a scanner reading a barcode

MHC I vs MHC II

MHC I - "The Normal Cell Bulletin Board"

Memory trick: MHC I loci have 1 letter each (A, B, C).

CD8 T cells (cytotoxic/killer T cells) read MHC I. Memory trick: 8 × 1 = 8 (CD8 × MHC1 = 8).

The molecule has a long heavy chain with 3 alpha domains, plus a separate shorter chain called β₂-microglobulin.

Every cell with a nucleus has MHC I. The only cells WITHOUT it are red blood cells (no nucleus). This makes sense - every cell needs to be able to show T cells what is happening inside.

• Endogenous = made inside the cell (endo = inner). If a virus infects a cell, it makes viral proteins inside. These proteins get chopped up and presented on MHC I.
• CD8+ T cells see this and say "this cell is infected!" → they kill it.
• Think: MHC I = inside the cell's story → presented to killer (CD8) T cells

• Proteins made inside the cell get chopped into small fragments (peptides) in the cytoplasm by the proteasome (a protein chopper)
• These fragments are transported into the RER (Rough Endoplasmic Reticulum) - a cellular compartment - via a protein transporter called TAP
• Inside the RER, the fragments get loaded onto MHC I molecules
• The whole package travels to the cell surface for display

This small protein stabilizes the MHC I structure. It is important in some cancers and diseases.

MHC II - "The Antigen Presenter's Board"

Memory trick: MHC II loci have 2 letters (DP, DQ, DR).

CD4 T cells (helper T cells) read MHC II. Memory trick: 4 × 2 = 8.

Unlike MHC I (on every cell), MHC II is only on cells that professionally present antigens. These are the specialized cells whose job is to sample the environment, pick up antigens, and show them to helper T cells.

• Exogenous = from outside the cell (exo = outside). When a macrophage eats a bacterium, it chops it up and presents the bacterial protein pieces on MHC II
• CD4+ helper T cells see this and get activated
• Think: MHC II = outside the cell's story (bacteria eaten from outside) → presented to helper (CD4) T cells

• The exogenous antigen is taken in by the APC and ends up in an endosome (a small vesicle/bubble inside the cell)
• This endosome acidifies (becomes acidic)
• Initially, a protein called invariant chain blocks the MHC II groove (to prevent it from picking up self-proteins in the RER)
• In the acidified endosome, the invariant chain gets cleaved, and the antigen fragment can load onto MHC II

HLA Subtypes Associated with Diseases

• Psoriatic arthritis
• Ankylosing spondylitis (a chronic spine arthritis causing a fused, rigid spine)
• IBD (Inflammatory Bowel Disease) associated arthritis
• Reactive arthritis (joint inflammation after a distant infection)

Memory: SL3 = SLE with DR3. And 1 + 3 = 4 (helps remember DR3 and DR4 for DM1 below)

Memory: "4 walls in 1 rheum(room)" - 4 walls in a room → DR4 → Rheumatoid arthritis

PAGE 6: NATURAL KILLER (NK) CELLS AND B & T CELL FUNCTIONS

Natural Killer (NK) Cells

NK cells look like lymphocytes (B and T cells) but behave like innate immunity - they don't need prior sensitization or specific antigen recognition to kill.

• Perforin = a protein that pokes holes in target cell membranes (like drilling a hole in a tank)
• Granzymes = enzymes that enter through the perforin holes and activate the cell's self-destruction machinery
• Apoptosis = programmed cell death (the cell kills itself in a controlled way - no inflammation, just clean death)
• NK cells kill virally infected cells AND cancer cells - a very important defense

These are cytokines (chemical messengers) that turbocharge NK cells:

• IL-2 = interleukin-2 (T-cell growth factor; also activates NK cells)
• IL-12 = made by macrophages; activates NK cells and promotes Th1 T-cell response
• IFN-α and IFN-β = Type I interferons made when cells detect viruses; they put neighboring cells on high alert AND activate NK cells

NK cells produce IFN-γ (Interferon-gamma) which activates macrophages to become more aggressive killers. This is a bridge between innate and adaptive immunity.

This is the key principle: "Missing self" hypothesis

• Normal cells display MHC I. NK cells have receptors that recognize MHC I and receive an inhibitory (stop) signal - "this cell is normal, don't kill it"
• Virally infected cells and cancer cells often downregulate (reduce or remove) MHC I so T cells can't recognize them. But this backfires - without MHC I, the NK cell loses its inhibitory signal and says "no ID = suspicious → kill!"
• It is a brilliant backup system: viruses hide from T cells by removing MHC I, but then NK cells kill them instead

• ADCC (Antibody-Dependent Cell-Mediated Cytotoxicity) = antibodies coat a target cell, and then NK cells bind to the antibody's Fc region (the tail of the antibody) via CD16 receptor and kill the cell. It is like a missile guided by an antibody
• CD16 = a receptor on NK cells that grabs IgG antibodies

MAJOR FUNCTIONS OF B CELLS

• B cells have B-cell receptors (BCRs) that bind antigens directly (unlike T cells that need MHC presentation)
• Somatic hypermutation = after activation, B cells in germinal centers mutate their antibody genes at an incredibly high rate. Each mutation slightly changes the antibody, and cells with BETTER-fitting antibodies survive (natural selection happening in real time!). This is how antibodies improve their fit over time = affinity maturation

Activated B cells transform into plasma cells = pure antibody factories. They do nothing else but produce thousands of antibodies per second.

Long-lived memory B cells survive after infection, sometimes for decades. This is why once you have measles, you are immune for life.

MAJOR FUNCTIONS OF T CELLS

Line: "Help B cells make antibodies and produce cytokines to recruit phagocytes and activate other leukocytes." CD4+ T cells are the commanders/coordinators. They:

1. Tell B cells to start making antibodies
2. Release cytokines to recruit more immune cells to the site of infection
3. Activate macrophages to become more powerful killers

Line: "Directly kill virus-infected and tumor cells via perforin and granzymes (similar to NK cells)." CD8 T cells are the assassins. Like NK cells, they use perforin + granzymes. But unlike NK cells, CD8 T cells are highly specific - each one only kills cells presenting a specific viral antigen on MHC I.

T cells are responsible for Type IV (delayed) hypersensitivity - reactions that take 48-72 hours (like tuberculin skin test/PPD test, contact dermatitis from poison ivy).

When you receive an organ transplant, T cells see the donor's MHC molecules as foreign and attack the organ. This is graft rejection. Anti-rejection medications mainly target T cells.

PAGE 7: DIFFERENTIATION OF T CELLS

T-Cell Precursor Journey

1. Precursors are double positive (CD4+ AND CD8+) initially
2. They then commit to being either CD8+ (cytotoxic) or CD4+ (helper)

Th1 cells

Made when IL-12 and IFN-γ are present (and NOT IL-4, IL-10)

• Secrete: IFN-γ, IL-2
• Function: Activate macrophages and cytotoxic T cells - mainly fight intracellular pathogens (bacteria living inside cells, like TB, Listeria)

Th2 cells

Made when IL-4 is present (and NOT IFN-γ)

• Secrete: IL-4, IL-5, IL-6, IL-10, IL-13
• Function: Activate eosinophils, promote IgE production - mainly fight parasites (worms) and mediate allergic reactions
• IL-5 specifically activates eosinophils (which fight worms and participate in allergies)
• IL-4 and IL-13 promote IgE (the allergy antibody)

Th17 cells

Made when TGF-β, IL-1, IL-6 are present (and NOT IFN-γ, IL-4)

• Secrete: IL-17, IL-21, IL-22
• Function: Induce neutrophilic infiltration - fight extracellular bacteria and fungi. Th17 deficiency leads to recurrent bacterial and fungal infections

Regulatory T cells (Tregs)

Made when TGF-β and IL-2 are present (and NOT IL-6)

• Secrete: TGF-β, IL-10, IL-35
• Function: Prevent autoimmunity (maintain tolerance) - they are the brakes of the immune system. Without Tregs, the immune system attacks the body itself

POSITIVE SELECTION - "Keep the Useful Ones"

In the thymic cortex, T cells must prove they can read MHC. If a T cell's receptor CAN recognize self-MHC, it gets a survival signal ("you're useful, stay alive"). If it CANNOT read MHC, it dies by neglect. This ensures all surviving T cells are capable of interacting with the body's own antigen-presenting machinery. Think: Positive selection = reading test. Pass the test = survive

NEGATIVE SELECTION - "Eliminate the Dangerous Ones"

After positive selection, T cells move to the medulla for a second test. If a T cell binds TOO STRONGLY to self-antigens, it is dangerous (it would attack the body's own tissues = autoimmunity). These cells are destroyed. Think: Negative selection = safety check. Too reactive = eliminated

• AIRE (AutoImmune REgulator) = a protein in the thymic medulla that allows the thymus to display almost any body protein (like thyroid protein, insulin, etc.) so T cells can be tested against them
• If AIRE is deficient, T cells that would attack the thyroid, adrenal glands, etc. are NOT eliminated → autoimmune attacks on multiple glands
• APS-1 (Autoimmune Polyendocrine Syndrome type 1) = the disease caused by AIRE deficiency, also called APECED
• Mnemonic: "Without AIRE, your body will CHAR" = Chronic mucocutaneous candidiasis, Hypoparathyroidism, Addison disease (adrenal), Recurrent Candida

MACROPHAGE-LYMPHOCYTE INTERACTION

Th1 cells essentially supercharge macrophages. IFN-γ makes macrophages better at killing bacteria, especially intracellular ones like TB.

CD40L (CD154) on T cells binds to CD40 on macrophages/APCs. This physical contact between cells also activates macrophages. (CD40-CD40L interaction is also critical for B-cell class switching.)

CYTOTOXIC T CELLS (CD8+)

Three targets: virally infected cells, cancer cells (neoplastic), and transplanted cells (from donors).

The granules are preformed and ready to fire - no delay needed.

CD8 acts like a locking mechanism - it stabilizes the connection between the cytotoxic T cell and the target cell displaying viral antigen on MHC I.

REGULATORY T CELLS (Tregs)

They are the off switch. Without them, both helper and cytotoxic T cells would go rogue.

• CD3 = all T cells
• CD4 = helper T cell marker (Tregs look like helper T cells)
• CD25 = IL-2 receptor (Tregs need IL-2 to survive)
• FOXP3 = the master transcription factor that makes a T cell become a Treg. Think of FOXP3 as the regulatory gene

IL-10 and TGF-β are the two main anti-inflammatory cytokines. They calm down the immune response.

IPEX = Immune dysregulation, Polyendocrinopathy, Enteropathy, X-linked

• Caused by FOXP3 gene mutation → no Tregs → uncontrolled autoimmunity
• Enteropathy = severe intestinal inflammation (damaged gut)
• Endocrinopathy = autoimmune attack on glands (diabetes, thyroiditis)
• X-linked = mainly affects male infants
• Characterized by nail dystrophy, dermatitis, autoimmune conditions
• It is a catastrophic disease in male infants - essentially no immune brakes

PAGE 8: T- AND B-CELL ACTIVATION

APCs (Antigen-Presenting Cells)

• Dendritic cells = best APCs; patrol tissues, pick up antigens, travel to lymph nodes
• Langerhans cells = dendritic cells in the skin specifically
• B cells and Macrophages = also present antigens

This is a critical concept. ONE signal is not enough. The immune system requires TWO signals to prevent accidental activation. This is a safety mechanism.

T-CELL ACTIVATION - Step by Step

The APC (e.g., dendritic cell) in a tissue eats a bacterium, chops it up, loads it onto MHC, then travels to the nearest lymph node to present it.

• Signal 1 = the specific antigen recognition. TCR on T cell reads the MHC-antigen complex.
• CD4+ T helper cells read MHC II (exogenous antigens - bacteria from outside)
• CD8+ cytotoxic T cells read MHC I (endogenous antigens - viral proteins from inside)
• Cross-presentation = some APCs can present exogenous antigens on MHC I too, activating CD8 cells against bacteria/vaccines - a special mechanism

• Signal 2 = the costimulatory signal - the confirmation signal that infection is real
• B7 (CD80/86) = on the APC surface. Only expressed when the APC has been activated by a true infection
• CD28 = on the T cell surface. It is the receptor for B7.
• When B7 binds CD28, the T cell gets the green light to proliferate and activate
• If Signal 1 occurs without Signal 2 (no B7-CD28 interaction), the T cell becomes anergic (permanently inactivated) - this is a self-tolerance mechanism

After both signals: helper T cells produce cytokines to coordinate the response; cytotoxic T cells go hunt for infected cells.

B-CELL ACTIVATION AND CLASS SWITCHING

CD4+ helper T cell must be activated first.

The B cell uses its receptor to grab and internalize the antigen.

The B cell now acts as an APC, presenting the antigen on its own MHC II to the already-activated helper T cell. This is the physical meeting point.

CD40-CD40L interaction = the key handshake between B cell and T helper cell. This direct cell-to-cell contact is required for B cells to:

• Undergo class switching (change which antibody type they make)
• Undergo somatic hypermutation
• Form germinal centers CD40L deficiency (X-linked Hyper-IgM syndrome) = B cells can make IgM but cannot switch to IgG, IgA, IgE because the CD40-CD40L handshake never happens.

Different cytokines tell the B cell which antibody class to produce:

• IL-4 → IgE (allergy antibody)
• TGF-β → IgA (mucosal antibody - found in saliva, tears, breast milk)
• Various cytokines → IgG (the main blood antibody)

The first antibody made is always IgM (the initial, pentameric, less specific antibody). Then, with T cell help and cytokines, B cells switch to making IgG, IgA, or IgE (class switching), and their antibodies improve in quality (affinity maturation through somatic hypermutation).

ANERGY

Anergy = immune cell paralysis. If a T or B cell sees its antigen but does NOT receive Signal 2 (no costimulatory signal), it becomes permanently unresponsive. This is how the body prevents autoimmunity for self-antigens that escape into circulation - there are no costimulatory signals in normal tissues, so self-reactive cells become anergic.

PAGE 9: ANTIBODY STRUCTURE AND FUNCTION

Basic Antibody Structure

• Two arms of the Y = Fab regions
• Stem of the Y = Fc region

The Fab Region (the antigen-grabbing arms)

The variable region of Fab is what actually grabs the antigen - like fingers grabbing something.

• Idiotype = the unique fingerprint of an antibody's antigen-binding site. Each B cell makes one specific shape of Fab - it can only bind one specific antigen
• This specificity is absolute - one B cell, one antibody, one target

The Fc Region (the stem - the "constant" part)

• Constant = does not vary; same structure in all antibodies of the same isotype
• Carboxy terminal = the tail end chemically
• Complement binding = IgM and IgG Fc regions can activate the complement cascade
• Carbohydrate side chains = sugar attachments that affect function
• Confers (determines) isotype = the Fc region determines what class the antibody is (IgG, IgM, IgA, IgD, IgE)

Generation of Antibody Diversity

• V (Variable), D (Diversity), J (Joining) = gene segments that get randomly combined like puzzle pieces
• RAG1 and RAG2 = enzymes that perform this recombination (Recombination Activating Genes)
• If RAG is deficient → SCID (Severe Combined ImmunoDeficiency) - no B or T cells at all

TdT = an enzyme that randomly adds extra DNA letters at the junctions between gene segments. This creates even MORE diversity - like randomly adding letters when assembling words.

Any heavy chain can pair with any light chain - this combinatorial diversity creates an enormous variety.

After B cell activation in germinal centers, mutations accumulate in the variable region. B cells with BETTER fitting antibodies are selected to survive → the antibodies improve over time.

The constant region (Fc) changes, switching the antibody class from IgM to IgG/IgA/IgE. The antigen-binding specificity (Fab) stays the same - only the "tool" (Fc) changes.

Three Main Antibody Functions (from the diagram)

Antibody physically blocks bacteria from sticking to body cells - like a bouncer blocking the door. "Antibody prevents bacterial adherence."

Antibody coats the pathogen (marks it with a flag). Macrophages and neutrophils have Fc receptors and eat opsonized (flagged) pathogens much more efficiently. "Antibody promotes phagocytosis by marking pathogens for destruction."

IgG and IgM bound to pathogens trigger the complement cascade → formation of the MAC (Membrane Attack Complex) - a molecular drill that punches holes in bacterial membranes, killing them. C3b (a complement fragment) also acts as an opsonin itself.

1. Lymphoid structures (primary and secondary organs)
2. Lymph node anatomy (follicle, medulla, paracortex)
3. Lymphatic drainage and clinical associations
4. Spleen and thymus structure and function
5. Innate vs adaptive immunity
6. MHC I and II (with HLA disease associations)
7. NK cells, B cells, and T cells
8. T-cell differentiation (Th1/Th2/Th17/Treg), positive/negative selection
9. T- and B-cell activation (the 2-signal model)
10. Antibody structure and generation of diversity