COMPLETE LECTURE: Immunology - Lymphoid Structures & Cellular Components

(First Aid Pages 94-102, explained for non-medical readers)

PAGE 94 - IMMUNE SYSTEM ORGANS

WHAT IS THE IMMUNE SYSTEM?

LINE: "1° organs: Bone marrow - immune cell production, B cell maturation"

• "Immune cell production" - all white blood cells (the soldiers of the immune system) are made here from stem cells (stem cells = blank cells that can become anything).
• "B cell maturation" - B cells are one type of immune cell. The "B" stands for Bone marrow, because that is where they are not just born but also fully trained/matured. They learn what is self (your own body) vs foreign (enemies) right here.

LINE: "Thymus - T cell maturation"

• T cells = another type of immune cell. "T" stands for Thymus - they are born in bone marrow but travel to the thymus to be trained. Think of bone marrow as the hospital where soldiers are born, and the thymus as the military academy where they learn combat.

LINE: "2° organs: Spleen, lymph nodes, tonsils, adenoids, appendix, Peyer patches"

• Spleen = large organ on the left side of your abdomen. Filters your blood.
• Lymph nodes = small bean-shaped glands scattered throughout your body (you can feel them under your armpits, neck, groin when you are sick). They filter lymph fluid.
• Tonsils = those two lumps at the back of your throat - yes, the ones that get infected (tonsillitis)!
• Adenoids = similar tissue but at the back of your nose.
• Appendix = yes, the same appendix that gets inflamed (appendicitis). It actually has immune tissue!
• Peyer patches = patches of immune tissue in the wall of your small intestine. They watch for invaders coming in with food.

LINE: "Allow immune cells to interact with antigen"

LYMPH NODE (detailed section)

LINE: "A 2° lymphoid organ that has many afferents, 1 or more efferents. Encapsulated, with trabeculae."

• 2° lymphoid organ = deployment station (as we said).
• Afferents = "afferent" means going toward something. Afferent lymphatic vessels bring lymph fluid into the lymph node. Many vessels bring fluid in - so many afferents.
• Efferents = "efferent" means going away from something. Only 1-2 vessels take lymph fluid out of the node. This is like a funnel - fluid comes in through many pipes and exits through a narrow one.
• Encapsulated = the lymph node is covered by a capsule (a fibrous coat like the skin of an orange). This gives it structure and protection.
• Trabeculae = extensions of the capsule that go inside the lymph node, dividing it into compartments (like the white inner dividers of an orange). Trabeculae = Latin for "little beams."

LINE: "Functions are nonspecific filtration by macrophages, circulation of B and T cells, and immune response activation."

• Nonspecific filtration by macrophages: Macrophages are big, hungry cells (macro = big, phage = eater). They sit inside lymph nodes and eat debris, bacteria, and dead cells that come in with lymph fluid. This filtration is "nonspecific" because macrophages eat anything suspicious without being picky.
• Circulation of B and T cells: Lymph nodes are like bus terminals - B and T cells are constantly cycling through, checking for enemies.
• Immune response activation: When T or B cells encounter their specific enemy (antigen) inside the lymph node, they get activated and start an immune response. This is why lymph nodes swell when you are sick - the immune cells inside are multiplying and fighting.

LINE: "Located in outer cortex: site of B-cell localization and proliferation. 1° follicles are dense and quiescent."

• 1° (Primary) follicles: Dense, tightly packed clusters of B cells that are quiescent = resting/sleeping. They have not yet seen their enemy antigen. Think of soldiers sitting quietly in the barracks.
• 2° follicles have pale central germinal centers and are active: When B cells encounter their antigen, they get activated and form a germinal center in the middle of the follicle. This center appears pale under a microscope because the cells are dividing rapidly and spreading out. This is where B cells multiply, mutate their antibody genes to improve their weapons, and become plasma cells (antibody-secreting factories).

LINE: "Medulla: consists of medullary cords (closely packed lymphocytes and plasma cells) and medullary sinuses. Medullary sinuses communicate with efferent lymphatics."

• Medullary cords: Dense strings of lymphocytes (immune cells) and plasma cells (antibody factories). Think of them as the production lines deep inside the factory.
• Medullary sinuses: Spaces/channels between the cords. Lymph fluid flows through these sinuses on its way to the efferent vessel. Macrophages sit here, grabbing pathogens from the passing lymph fluid.
• Communicate with efferent lymphatics: The sinuses connect to the exit vessel, so filtered, cleaned lymph fluid can leave the node.

LINE: "Paracortex: contains T cells. Region of cortex between follicles and medulla. Contains high endothelial venules through which T and B cells enter from blood."

• High endothelial venules (HEV): Special blood vessels with tall (high) endothelial cells (cells lining the vessel wall). These are the doorways through which T and B cells enter the lymph node from the bloodstream. Regular blood vessels have flat, thin endothelial cells; HEVs have tall, plump ones that allow lymphocytes to squeeze through (this process is called diapedesis = cells crawling out of blood vessels).
• Underdeveloped in DiGeorge syndrome: DiGeorge syndrome = a genetic condition where the thymus is absent or underdeveloped. Because T cells cannot mature without a thymus, the paracortex (T cell zone) is empty and underdeveloped. This is clinically important - if you see a lymph node biopsy with an empty paracortex, think DiGeorge.
• Paracortex enlarges in extreme cellular immune response (e.g., EBV and other viral infections): EBV = Epstein-Barr Virus, the virus that causes mono (kissing disease). Viral infections activate T cells massively, so the paracortex (T cell zone) swells up. This causes the dramatic lymph node enlargement (lymphadenopathy = swollen lymph nodes) seen in mono.

PAGE 95 - LYMPHATIC DRAINAGE ASSOCIATIONS

Key lymph node groups and what they drain:

• Associated pathology: cancers of the head/neck, throat infections
• If a neck lymph node is swollen and hard, think cancer of the head, neck, or mouth

• Left supraclavicular node = Virchow's node - famously drains from the thorax (chest) AND abdomen (via the thoracic duct). A swollen left supraclavicular node = Troisier's sign = think abdominal malignancy, especially stomach cancer!
• Right supraclavicular node drains right lung, esophagus

• Drain the arm, upper chest wall, and - most importantly - the breast
• Associated pathology: breast cancer - this is why surgeons check axillary nodes in breast cancer surgery
• Also: melanoma of the upper limb

• Drain the lungs
• Associated pathology: sarcoidosis, primary TB, lung cancer, fungal infections

• Drain the heart, lungs, trachea, esophagus
• Pulmonary TB, lymphoma

• Drain testes, ovaries, kidneys, uterus, adrenal glands
• Testicular cancer typically goes to para-aortic nodes first (NOT inguinal nodes - this is a common exam trick! The testes develop in the abdomen and descend, so they keep their original lymphatic connections)

• Stomach cancer (gastric cancer) → metastasis here

• Mesenteric inflammatory bowel disease

• Colon cancer

• Colorectal cancer, cervical cancer, prostate cancer

• Sexually transmitted infections, melanoma of the lower limb

• Drain the foot and lateral lower leg
• Sexually transmitted infections (lateral foot/posterior calf infections)

KEY EXAM POINT: Right lymphatic duct vs thoracic duct

• Right lymphatic duct drains the right side of the body above the diaphragm and upper into the right subclavian vein
• Thoracic duct (left side) drains everything else - both legs, abdomen, left arm, left chest - into the junction of left subclavian and left internal jugular veins. This is why Virchow's node (left supraclavicular) can be enlarged by abdominal cancers - the thoracic duct deposits tumor cells nearby.

PAGE 96 - SPLEEN & THYMUS

SPLEEN

LINE: "Located in LUQ of abdomen, anterolateral to left kidney, protected by 9th-11th ribs."

• Anterolateral to left kidney = in front of and to the side of the left kidney
• Protected by 9th-11th ribs = the spleen is tucked under your lower left ribs. This protection is important - a broken rib (rib fracture) can pierce and rupture the spleen, causing dangerous internal bleeding.

RED PULP vs WHITE PULP

• This is where old, damaged, or deformed red blood cells are trapped and destroyed by macrophages. Think of it as a blood recycling center.
• Contains sinusoids (special blood vessels) and a reticular fibrous framework (scaffolding)
• Red because it is full of red blood cells
• Also has open circulation (blood flows freely through the tissue) and closed circulation (blood goes through actual vessel channels)

• Follicle (B cells) = B cell zone
• Mantle zone = ring of B cells around the follicle
• Germinal center = activated B cells dividing and making antibodies
• Marginal zone = explained below
• Periarteriolar lymphatic sheath (PALS) = T cell zone, wrapped around the central artery like a sleeve

LINE: "Periarteriolar lymphatic sheath: Contains T cells. Located within white pulp."

LINE: "Follicle: Contains B cells. Located within white pulp."

LINE: "Marginal zone: Contains macrophages and specialized B cells. Site where antigen-presenting cells capture blood-borne antigens for recognition by lymphocytes. Located between red pulp and white pulp."

• Macrophages here capture pathogens floating in blood (especially encapsulated bacteria - bacteria surrounded by a sugar capsule, like Streptococcus pneumoniae).
• Specialized B cells here respond to T-cell-independent antigens (polysaccharides/sugars on bacterial capsules). This is why asplenic patients (those without a spleen) are vulnerable to encapsulated bacteria.

SPLENIC DYSFUNCTION (Hyposplenism / Asplenia)

LINE: "Splenic dysfunction (eg, postsplenectomy, sickle cell disease autosplenectomy) → ↓ IgM → ↓ complement activation → ↓ C3b opsonization → ↑ susceptibility to encapsulated organisms, against which patients should be vaccinated from least common: pneumococci, meningococci, Haemophilus influenzae type b (Hib)."

1. Spleen removed or damaged (postsplenectomy = after spleen surgery; sickle cell autosplenectomy = in sickle cell disease, the spleen destroys itself because sickling damages it repeatedly over years)
2. ↓ IgM = Less IgM produced. IgM is the first antibody made against new infections. The spleen is the primary producer of IgM. Without a spleen, IgM levels fall.
3. ↓ Complement activation = Complement = a system of proteins in blood that act like chemical weapons. IgM activates the complement system. Less IgM = less complement activation.
4. ↓ C3b opsonization = Opsonization = coating a pathogen to make it easier for macrophages to eat. C3b is a complement protein that sticks to bacteria, flagging them as "eat me." Less C3b = bacteria escape being eaten.
5. ↑ Susceptibility to encapsulated organisms = Encapsulated bacteria (those with a sugar capsule) are especially dangerous because their capsule makes them resistant to phagocytosis (being eaten). The spleen and its IgM/C3b system is the main defense against these bacteria. Without a spleen, you are dangerously vulnerable to:
   • Pneumococci (Streptococcus pneumoniae) - causes pneumonia, meningitis
   • Meningococci (Neisseria meningitidis) - causes meningitis
   • H. influenzae type b (Hib) - causes meningitis, epiglottitis
   • Remember the mnemonic: SHiN (S. pneumoniae, H. influenzae, N. meningitidis)
6. Patients should be vaccinated against all three before splenectomy if possible.

POSTSPLENECTOMY FINDINGS (what you see in blood after spleen is removed)

• Howell-Jolly bodies = nuclear remnants (tiny purple dots) inside red blood cells. Normally the spleen removes these; without a spleen, they accumulate. Seeing Howell-Jolly bodies on a blood smear = think asplenia.
• Target cells = red blood cells with a bullseye appearance. Seen in liver disease, asplenia, hemoglobin disorders.
• Thrombocytosis = too many platelets (thrombocytes). The spleen normally sequesters (stores) about 1/3 of your platelets. Without the spleen, platelets flood the blood.
• Lymphocytosis = too many lymphocytes in blood (loss of sequestration - spleen normally holds lymphocytes too).

THYMUS

LINE: "Located in the anterosuperior mediastinum."

LINE: "Site of T-cell differentiation and maturation."

LINE: "Encapsulated. Thymus epithelium is derived from third pharyngeal pouch (endoderm), whereas thymic lymphocytes are of mesodermal origin."

• Third pharyngeal pouch = during embryo development, the throat region has pouches that develop into different structures. The 3rd pouch gives rise to the thymus and inferior parathyroid glands. This is why in DiGeorge syndrome (failure of the 3rd pharyngeal pouch to develop), the thymus AND parathyroid glands are both absent.
• Endoderm = the inner germ layer of the embryo (forms internal organs, glands)
• Mesodermal origin = the T cells (lymphocytes) in the thymus come from a different embryonic layer (mesoderm). So the "building" (thymus epithelium) is from endoderm, but the "tenants" (T cells) are from mesoderm.

LINE: "Cortex is dense with immature T cells; medulla is pale with mature T cells and Hassall corpuscles containing epithelial reticular cells."

• Cortex (outer zone) = packed with immature T cells (thymocytes) undergoing training/selection. Dense and dark because so many cells are crowded together.
• Medulla (inner zone) = mature T cells that have passed selection. Fewer cells, so appears pale.
• Hassall corpuscles = unique structures in the thymic medulla - concentric whorls (spiraling rings) of dead epithelial reticular cells. They are like the "waste disposal" areas. Unique to the thymus. Seen only in the thymus medulla = a histology exam favorite!

LINE: "Normal neonatal thymus 'sail-shaped' on CXR (arrows in A), involutes by age 3 years."

• Neonatal = newborn. In babies, the thymus is large and appears as a wide sail shape on chest X-ray (CXR). This is completely normal - don't misinterpret it as a mass.
• Involutes by age 3 = the thymus shrinks (involutes) rapidly through childhood. By puberty it is largely replaced by fat.
• Absent thymic shadow or hypoplastic thymus = seen in DiGeorge syndrome or SCID (severe combined immunodeficiency) - a diagnostic clue on chest X-ray.

LINE: "T cells = Thymus, B cells = Bone marrow"

LINE: "Thymoma - neoplasm of thymus. Associated with myasthenia gravis, superior vena cava syndrome, pure red cell aplasia, Good syndrome."

• Thymoma = cancer/tumor of the thymus.
• Myasthenia gravis = autoimmune disease where antibodies attack neuromuscular junctions, causing muscle weakness. Strongly associated with thymoma - removing the thymoma often helps.
• Superior vena cava syndrome = the tumor compresses the SVC (superior vena cava = the large vein returning blood from upper body to heart), causing facial swelling, arm swelling, and distended neck veins.
• Pure red cell aplasia = bone marrow stops making red blood cells.
• Good syndrome = thymoma + immunodeficiency (low immunoglobulins).

PAGE 97 - INNATE vs ADAPTIVE IMMUNITY

THE BIG PICTURE FIRST

1. Innate immunity = the fast, non-specific, first responder. Like a security guard at the door who tackles any suspicious person immediately.
2. Adaptive immunity = the slow, highly specific, trained specialist. Like a detective who studies each criminal individually and creates a tailored response.

COMPONENTS

• Neutrophils = the most abundant white blood cell; first to arrive at infections; they eat bacteria and release toxic chemicals. Live only hours.
• Macrophages = big eater cells (macro = big, phage = eater). They eat pathogens and also activate T cells by presenting antigen.
• Monocytes = precursors of macrophages in blood. They circulate in blood, then enter tissues and become macrophages.
• Dendritic cells = have long projections like a spider. They are the best antigen-presenting cells (APCs) - they capture antigens in tissues, then travel to lymph nodes and present them to T cells. They are the messengers that connect innate and adaptive immunity.
• NK cells = Natural Killer cells. Lymphoid origin but part of innate immunity. They kill virus-infected cells and tumor cells WITHOUT needing prior sensitization (no memory required).
• Complement = a cascade of ~30 proteins in blood that are activated like falling dominoes. They punch holes in bacteria, attract immune cells, and flag pathogens for destruction.
• Physical epithelial barriers = your skin, mucus, cilia, stomach acid - physical/chemical walls that prevent pathogens from entering.
• Secreted enzymes = lysozyme in tears/saliva (digests bacterial cell walls), defensins, stomach acid.

MECHANISM

• "Germline encoded" means the recognition patterns are built into the DNA from birth. You are born knowing these patterns - no learning required. Every innate immune cell automatically recognizes the same danger patterns.

• V(D)J recombination = a genetic shuffling process. Like shuffling a deck of cards, genes for immune receptors are randomly rearranged to create billions of different receptor shapes. This is how you can recognize virtually any pathogen, even ones never seen before. Each lymphocyte (B or T cell) gets a unique receptor. This process happens during development in bone marrow/thymus.

RESPONSE TO PATHOGENS

• "Nonspecific" = same response to all invaders. Within minutes of a bacterium entering your skin, neutrophils are already at the site. No need for prior exposure.
• "No memory" = innate immunity does not remember past infections. It responds the same way every time.

• "Specific" = each B and T cell is tailored to one specific antigen, like a key fitting a specific lock.
• "Memory response" = the first time you encounter a pathogen (primary response), it takes 1-2 weeks to mount a full response. But memory B and T cells persist and when you encounter the same pathogen again, the response is faster and stronger (secondary response). This is the principle behind vaccines.
• "Adaptive immune responses decrease with age (immunosenescence)" = as you age, adaptive immunity weakens. Elderly people respond less strongly to vaccines and new infections.

SECRETED PROTEINS

• Lysozyme = enzyme that destroys bacterial cell walls. Found in tears, saliva, mucus.
• C-reactive protein (CRP) = a protein made by the liver during inflammation. Rises within hours of infection/injury. A non-specific marker of inflammation - commonly measured in blood tests. High CRP = something is inflamed somewhere.
• Defensins = small proteins that punch holes in bacterial membranes. Found on mucosal surfaces.
• Cytokines = chemical messenger molecules released by immune cells to communicate with each other (like texting between soldiers). Examples: interleukins (IL-1, IL-6, etc.), interferons, tumor necrosis factor (TNF).

• Immunoglobulins = antibodies. These are the specific weapons of adaptive immunity - Y-shaped proteins that bind exactly one antigen.

KEY FEATURES IN PATHOGEN RECOGNITION

• TLRs = pattern recognition receptors. They recognize PAMPs and DAMPs.
• PAMPs = Pathogen-Associated Molecular Patterns. These are molecular patterns found on pathogens but NOT on normal human cells. They are evolutionary conserved signals that say "I am foreign/dangerous."
  • Examples: LPS (lipopolysaccharide) on gram-negative bacteria's outer membrane, flagellin (the protein making bacterial flagella/tails), nucleic acids from viruses.
• DAMPs = Damage-Associated Molecular Patterns. These are signals released by your own cells when they are damaged or dying - they scream "there is danger here!"
  • Examples: mitochondrial DNA, histones (proteins that pack DNA), heat shock proteins (released by stressed cells).
• When TLR binds PAMP/DAMP → activation of NF-kB → release of pro-inflammatory cytokines:
  • NF-kB = Nuclear Factor kappa B. It is a transcription factor (a molecular switch inside the cell nucleus) that turns on genes for making inflammatory cytokines. Think of it as the "emergency broadcast" button inside immune cells.

• Activated B and T cells; subsequent exposure to previously encountered antigen → stronger, quicker immune response.
• This is your immune memory. Memory cells patrol your body for years (sometimes lifetimes), ready to respond the moment they see their specific enemy again.
• "Adaptive immune responses decrease with age (immunosenescence)" = elderly patients have weaker memory responses and lower vaccine effectiveness.

IMMUNE PRIVILEGE

LINE: "Organs (eg, eye, brain, placenta, testes) and tissues where chemical or physical mechanisms limit immune responses to foreign antigens to avoid damage that would occur from inflammatory sequelae. Allograft rejection at these sites is less likely."

• Eye = inflammation inside the eye could destroy vision. So the eye uses physical barriers (blood-ocular barrier) and secretes anti-inflammatory molecules.
• Brain = inflammation causes brain swelling, seizures, and death. The blood-brain barrier (BBB) restricts immune cell entry.
• Placenta = the fetus is half "foreign" (contains paternal genes). If the immune system attacked it, every pregnancy would be rejected. The placenta secretes immunosuppressive molecules.
• Testes = sperm are produced after the immune system is established. Sperm have new proteins the immune system has never seen. A "blood-testis barrier" prevents immune cells from seeing sperm.

PAGE 98 - MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) I AND II

WHAT IS MHC? (Explain to a Non-Medico)

MHC CLASS I

MHC CLASS II

HLA SUBTYPES ASSOCIATED WITH DISEASES

• Mnemonic: PAIR (Psoriatic arthritis, Ankylosing spondylitis, IBD arthritis, Reactive arthritis)
• Ankylosing spondylitis = chronic inflammation of the spine, causing it to fuse (ankylos = fused).
• Reactive arthritis = arthritis triggered by an infection elsewhere (formerly "Reiter's syndrome"): urethritis + conjunctivitis + arthritis = "can't pee, can't see, can't climb a tree."

• Mnemonic: DM type 1 is HLA-3 and -4 (1+3=4)
• DM type 1 = Type 1 diabetes - autoimmune destruction of insulin-producing cells
• SLE = Systemic Lupus Erythematosus - autoimmune attack on multiple organs
• Graves disease = autoimmune hyperthyroidism
• Hashimoto thyroiditis = autoimmune hypothyroidism
• Addison disease = autoimmune adrenal gland failure

• Mnemonic: There are 4 walls in 1 "rheum" (room) - for Rheumatoid arthritis

PAGE 99 - NK CELLS, B CELLS, T CELLS

NATURAL KILLER (NK) CELLS

LINE: "Lymphocyte member of innate immune system."

LINE: "Use perforin and granzymes to induce apoptosis of virally infected cells and tumor cells."

• Perforin = a protein that punches a pore (hole) in the target cell membrane - like drilling a hole in a wall.
• Granzymes = enzymes that enter through the perforin hole and trigger apoptosis (programmed cell death = the cell commits suicide in a controlled way). Apoptosis = the cell essentially follows instructions to self-destruct cleanly, without spilling its contents (which would cause inflammation).

LINE: "Activity enhanced by IL-2, IL-12, IFN-α, and IFN-β. Produce IFN-γ → macrophage activation."

• IL-2 = Interleukin-2. A growth/activation signal for lymphocytes. Produced by T cells.
• IL-12 = made by macrophages and dendritic cells; activates NK cells and promotes Th1 responses.
• IFN-α and IFN-β = type I interferons, produced when a cell is infected by virus. They "interfere" with viral replication and also activate NK cells.
• IFN-γ (interferon-gamma) = produced by NK cells; activates macrophages to kill better. A critical link between NK cells and macrophages.

LINE: "Induced to kill when exposed to a nonspecific activation signal on target cell and/or to an absence of an inhibitory signal such as MHC I on target cell surface."

• Normal cells display MHC I = "I am a healthy self cell, don't kill me" = NK cell is inhibited.
• Virus-infected or tumor cells often downregulate MHC I (they hide their ID badge to escape cytotoxic T cells). But in doing so, they accidentally trigger NK cells!
• NK cell logic: "Missing MHC I = something is wrong with this cell. Kill it!"
• This is called "missing self hypothesis" - NK cells kill cells that are missing their identity badges.

LINE: "Also kills via antibody-dependent cell-mediated cytotoxicity (ADCC). CD16 binds Fc region of bound IgG, activating the NK cell."

• ADCC = another killing mechanism. When antibodies (IgG) coat a target cell, they stick their Fc (tail) portion outward. NK cells have a receptor called CD16 (also called FcγRIII) that grabs this Fc tail. This docking activates the NK cell to kill the antibody-coated target. It's teamwork between antibody (adaptive) and NK cell (innate).
• Fc region = the tail/stem of the antibody Y-shape. (Fab = arms that bind antigen; Fc = tail that binds complement and Fc receptors).

MAJOR FUNCTIONS OF B CELLS

LINE: "Humoral immunity."

LINE: "Recognize and present antigen - undergo somatic hypermutation to optimize antigen specificity."

• B cells have B cell receptors (BCRs) on their surface that recognize antigens.
• Somatic hypermutation = after being activated in germinal centers (remember those pale centers in lymph node follicles?), B cells deliberately introduce random mutations into their antibody genes at a very high rate (hypermutation). The B cells that happen to produce a better-fitting antibody are selected and survive. This process is called affinity maturation - the antibody's grip on the antigen gets stronger and stronger over time. It is like evolution happening inside one organ, in one week.

LINE: "Produce antibody - differentiate into plasma cells to secrete specific immunoglobulins."

• Plasma cells = the final form of an activated B cell. A plasma cell is essentially an antibody factory - it no longer circulates or fights, it just produces thousands of antibody molecules per second. Plasma cells live in bone marrow and secrete antibodies for years.

LINE: "Maintain immunologic memory - memory B cells persist and accelerate future response to antigens."

• After the infection is cleared, most plasma cells die, but a small population of memory B cells persists in lymph nodes and bone marrow for years/decades (sometimes life). They are primed to respond instantly to re-exposure.

MAJOR FUNCTIONS OF T CELLS

LINE: "Cell-mediated immunity."

LINE: "CD4+ T cells help B cells make antibodies and produce cytokines to recruit phagocytes and activate other leukocytes."

• CD4+ = Helper T cells (Th). "CD4+" is the surface marker. These cells are the orchestrators of the entire immune response.
• They help B cells: by providing the CD40L signal and cytokines needed for B cell activation and class switching.
• They produce cytokines: these recruit more immune cells to the site (like calling for backup).
• They are the cells destroyed by HIV - which is why HIV patients get severe immunodeficiency.

LINE: "CD8+ T cells directly kill virus-infected and tumor cells via perforin and granzymes (similar to NK cells)."

• CD8+ = Cytotoxic T cells (Tc). They directly kill cells that present foreign antigen on MHC I.
• They use the same machinery as NK cells (perforin + granzymes) but are much more specific - they only kill cells presenting their exact antigen.

LINE: "Type IV hypersensitivity reaction."

• T cells (CD4+) mediate Type IV hypersensitivity = delayed-type hypersensitivity. This occurs 48-72 hours after antigen exposure (not immediate like allergic reactions). Examples: tuberculin skin test (PPD test for TB), contact dermatitis (poison ivy), nickel allergy. The delay is because T cells need time to arrive at the site.

LINE: "Acute and chronic cellular organ rejection."

• When a transplanted organ is rejected, T cells are the main culprits. They recognize foreign MHC as "not self" and attack the organ. This is why transplant patients take immunosuppressive drugs that primarily target T cells (eg, cyclosporine, tacrolimus).

PAGE 100 - T CELL DIFFERENTIATION

THE STORY OF T CELL DEVELOPMENT

POSITIVE SELECTION (Thymic Cortex)

LINE: "Thymic cortex. Keeps T cells that recognize self-peptides to allow for cooperation in immune responses. Double positive thymocytes expressing TCRs that recognize self-peptide MHC complexes receive a survival signal."

• Happens in the cortex (outer zone) of the thymus.
• Thymic cortical cells (epithelial cells) present self-peptides on MHC to developing T cells.
• If the T cell's receptor (TCR) can weakly bind to self-MHC → it receives a survival signal → it lives! (Positive selection = selected to survive)
• If the T cell's receptor cannot bind self-MHC at all → it dies (no survival signal = neglect death). About 95% of T cells die here.
• Why? Because a T cell that cannot recognize MHC is useless - it will never be able to interact with any cell in your body.
• Double positive (CD4+CD8+) cells become either:
  • Single positive CD4+ (if they bound to MHC II)
  • Single positive CD8+ (if they bound to MHC I)

NEGATIVE SELECTION (Thymic Medulla)

LINE: "Thymic medulla. Removes T cells that bind too strongly to self-peptides. Thymocytes expressing TCRs with high affinity for self-antigens undergo apoptosis or become regulatory T cells. The autoimmune regulator (AIRE) protein drives negative selection, and deficiency leads to autoimmune polyendocrine syndrome..."

• Happens in the medulla (inner zone) of the thymus.
• AIRE (AutoImmune REgulator) = a protein expressed by thymic medullary cells. AIRE is remarkable - it forces medullary cells to display proteins normally found only in the pancreas, thyroid, liver, etc. (peripheral tissue proteins). This way, T cells get to "see" tissue-specific proteins while still in the thymus.
• If a T cell binds too strongly to these self-peptides → danger! It would attack your own organs (autoimmunity) → so it is killed (apoptosis) or converted to a regulatory T cell.
• AIRE deficiency → APS-1 (Autoimmune PolyEndocrine Syndrome Type 1) = APECED:
  • Chronic mucocutaneous candidiasis (fungal infections of mucous membranes and skin)
  • Hypoparathyroidism (low calcium)
  • Adrenal insufficiency (Addison disease)
  • Recurrent Candida infections
  • Mnemonic: "Without AIRE, your body will CHAR"

T HELPER CELL SUBTYPES (Th1, Th2, Th17, Treg)

• Th1 = the general who commands the cell-killing tanks (macrophages, cytotoxic T cells). Good for viruses and bacteria inside cells.
• Th2 = the general who commands the allergy brigade (IgE, eosinophils). Good for parasites; bad when it overreacts (asthma, atopy).
• Th17 = calls for neutrophil (pus cell) reinforcements. Important for fungal defense.
• Treg = the peacekeeper who stops the army from attacking its own country.

MACROPHAGE-LYMPHOCYTE INTERACTION

LINE: "Th1 cells secrete IFN-γ, which enhances the ability of monocytes and macrophages to kill microbes they ingest. This function is also enhanced by interaction of T cell CD40L with CD40 on macrophages. Macrophages also activate lymphocytes via antigen presentation."

• IFN-γ from Th1 cells = supercharges macrophages. A normal macrophage eats bacteria but may not kill them all. IFN-γ makes the macrophage more efficient at killing - it upregulates the macrophage's toxic killing machinery (reactive oxygen species, nitric oxide).
• CD40L (on T cells) + CD40 (on macrophages) = direct cell-to-cell contact that also activates macrophages. This is the same CD40/CD40L interaction involved in B cell activation.
• Bidirectional relationship: T cells activate macrophages (via IFN-γ and CD40L) AND macrophages activate T cells (via antigen presentation on MHC II and co-stimulatory signals).

CYTOTOXIC T CELLS

LINE: "Kill virus-infected, neoplastic, and donor graft cells by inducing apoptosis. Release cytotoxic granules containing preformed proteins (eg, perforin, granzyme B). Cytotoxic T cells have CD8, which binds to MHC I on virus-infected cells."

• Neoplastic = cancer cells.
• Donor graft cells = transplanted organ cells (foreign MHC triggers CD8+ T cells).
• Preformed proteins = the granules are already loaded with perforin and granzyme B, so killing is rapid once the T cell docks.
• CD8 binds MHC I: This is the lock-and-key interaction. CD8 stabilizes the TCR-MHC I interaction so the T cell can read the antigen presented.

REGULATORY T CELLS (Tregs)

LINE: "Help maintain specific immune tolerance by suppressing CD4+ and CD8+ T-cell effector functions."

LINE: "Identified by expression of CD3, CD4, CD25, and FOXP3."

• CD3 = all T cells have CD3 (the signaling component of the TCR complex)
• CD4 = Tregs are CD4+
• CD25 = the receptor for IL-2; marks activated/regulatory T cells
• FOXP3 = the master transcription factor that makes a T cell become a Treg. If FOXP3 is absent → no Tregs → autoimmunity.

LINE: "IPEX (Immune dysregulation, Polyendocrinopathy, Enteropathy, X-linked) syndrome - genetic deficiency of FOXP3 → autoimmunity."

• IPEX = a severe disease of infant boys (X-linked, so affects males)
• No FOXP3 = No Tregs = the immune system goes haywire and attacks everything
• Characterized by: Enteropathy (severe gut disease), endocrinopathy (multiple gland failures), and autoimmune skin conditions
• Presents in male infants with intractable diarrhea, diabetes, and eczema

PAGE 101 - T AND B CELL ACTIVATION + ANERGY

APCs (Antigen-Presenting Cells)

LINE: "APCs: B cells, dendritic cells, Langerhans cells, macrophages."

• Langerhans cells = specialized dendritic cells found in the skin. They are the sentinels of the skin - capturing antigens that penetrate through skin.
• All APCs present antigen on MHC II to CD4+ T cells.

TWO SIGNALS ARE REQUIRED FOR T-CELL ACTIVATION

T-CELL ACTIVATION (step by step)

1. APC (eg, dendritic cell) ingests an antigen (like a bacterium) in peripheral tissue, then migrates to the draining lymph node.

1. Activated Th cell produces cytokines. Cytotoxic T cell can now recognize and kill virus-infected cells.

B-CELL ACTIVATION AND CLASS SWITCHING (step by step)

1. Th-cell activation as above (T cell gets activated first).
2. B-cell receptor-mediated endocytosis = the B cell uses its B cell receptor (BCR = surface antibody) to grab an antigen and pull it inside (endocytosis).
3. The antigen is processed and presented on MHC II to the Th cell. The B cell acts as an APC here.
4. CD40 receptor on B cell binds CD40L (CD40 ligand) on activated Th cell = direct contact between T and B cell. This is the critical "help" signal. Without CD40-CD40L interaction → B cell cannot class switch or undergo somatic hypermutation. (Important: CD40L deficiency = Hyper-IgM syndrome = B cells can only make IgM, cannot switch to IgG/IgA/IgE).
5. Th cells secrete cytokines that determine Ig class switching = the type of cytokine produced by Th cells determines what antibody class (IgG, IgA, IgE) the B cell switches to. For example: IL-4 promotes IgE production (relevant in allergy/asthma).
6. B cells are activated and produce IgM first (default). They then undergo class switching (change from IgM to IgG, IgA, or IgE) and affinity maturation (improve antibody quality through somatic hypermutation in germinal centers).

ANERGY

LINE: "State during which a cell cannot become activated by exposure to its antigen. T and B cells become anergic when exposed to their antigen without costimulatory signal (signal 2). Another example of peripheral tolerance mechanism."

• If a T cell sees its antigen (signal 1) but does NOT receive the co-stimulatory signal (signal 2/CD28-B7 interaction) → it becomes anergic = permanently inactivated/paralyzed. It cannot respond even if it later receives proper stimulation.
• This is a peripheral tolerance mechanism = a way to prevent self-reactive T cells (that escaped thymic deletion) from causing autoimmunity in the periphery.
• Why does this protect against autoimmunity? Self-antigens are displayed by cells that do NOT express B7 (co-stimulatory molecules). Only professional APCs (dendritic cells) that have been activated by danger signals express B7. So if a T cell sees a self-antigen on a normal cell (no B7) → anergy, no attack.
• This is also why tumors can suppress the immune response - tumor cells downregulate B7 so T cells become anergic even when they see tumor antigens. Checkpoint inhibitors (cancer drugs like pembrolizumab) work by restoring this co-stimulatory pathway.

PAGE 102 - ANTIBODY STRUCTURE AND IMMUNE RESPONSES

ANTIBODY STRUCTURE

• 2 heavy (H) chains = the long arms and stem of the Y
• 2 light (L) chains = smaller chains on the outside of the arms

REGIONS OF THE ANTIBODY:

• The arms of the Y
• Contains VARIABLE regions (the unique, antigen-specific part)
• 1 Fab per B cell = each B cell makes ONE type of antibody
• Determines idiotype = the unique antigen-binding identity of the antibody
• "Determines idiotype: unique antigenic specificity" - the variable region is so unique it can itself be recognized as an antigen by other antibodies

• The stem/tail of the Y
• Contains CONSTANT regions (same in all antibodies of the same class)
• Carboxy terminal
• Complement binding = Fc activates complement (specifically IgM and IgG fix complement via the classical pathway)
• Macrophage binding = macrophages and NK cells have Fc receptors (FcR) that grab the Fc tail of antibody-coated targets (this is how opsonization and ADCC work)
• Carbohydrate side chains = sugar groups attached to the Fc region
• Confers (determines) isotype = the Fc region determines whether the antibody is IgG, IgA, IgM, IgD, or IgE
• "IgM and IgG fixes complement" = specifically these two classes activate the classical complement pathway

GENERATION OF ANTIBODY DIVERSITY:

1. V(D)J recombination = Random shuffling of V (Variable), D (Diversity), and J (Joining) gene segments. This is done by enzymes encoded by RAG1 and RAG2 genes. Like shuffling 3 decks of cards independently and then combining one card from each deck.
2. Random addition of nucleotides = extra random nucleotides are added at the V-D and D-J junctions during recombination by the enzyme TdT (terminal deoxynucleotidyl transferase). This creates enormous extra diversity.
3. Random combination of heavy and light chains = each antibody has one heavy and one light chain. The combination of any heavy chain with any light chain multiplies the diversity enormously.
4. Somatic hypermutation and affinity maturation = after antigen exposure, random mutations accumulate in the variable region (in germinal centers). Better-fitting antibodies are selected. This is antigen-DEPENDENT diversity (happens only after exposure to antigen).
5. Isotype switching (class switching) = same variable region (same antigen specificity) is attached to a different constant region. The antibody keeps the same "targeting system" but gets a different "weapon." Also antigen-dependent (requires T cell help).

THREE FUNCTIONS OF ANTIBODIES:

1. Neutralization = antibody physically blocks a pathogen from entering cells. Like putting a cork in the pathogen's docking site. Prevents bacterial adhesion and viral entry. IgA does this at mucosal surfaces (gut, respiratory tract).
2. Opsonization = antibody coats the pathogen, promoting phagocytosis. Macrophages have Fc receptors - they grab the Fc tail of the antibody and eat the pathogen. The pathogen is tagged "eat me." IgG is the main opsonizing antibody.
3. Complement activation = IgM and IgG bound to a pathogen activate the complement cascade (classical pathway). Complement then:
   • Forms the MAC (Membrane Attack Complex) = punches holes in bacterial membranes
   • Deposits C3b (opsonization)
   • Releases C3a/C5a (attract neutrophils = chemotaxis)

IMMUNOGLOBULIN ISOTYPES (IgG, IgA, IgM, IgD, IgE)

IgG:

• Most abundant antibody in blood
• Can cross the placenta (provides maternal immunity to fetus)
• Main antibody of secondary immune response
• Opsonizes, fixes complement, crosses into tissues

IgA:

• Found in secretions (tears, saliva, breast milk, mucus, gut secretions)
• Exists as a dimer (J-chain holds 2 IgA molecules together)
• First line of mucosal defense - neutralizes pathogens at body surfaces
• Prevents bacterial adherence to mucosal surfaces

IgM:

• Pentamer (5 antibody units linked by J-chain) = 10 antigen-binding sites
• First antibody made in any immune response (primary response)
• Strongest complement activator
• Does NOT cross the placenta
• ABO blood group antibodies are IgM

IgD:

• Found on the surface of naive B cells along with IgM
• Acts as a B cell receptor signaling molecule
• Very low serum levels; function not fully understood

IgE:

• Very low serum levels normally
• Binds to mast cells and basophils (via Fc receptors on these cells)
• When antigen cross-links two IgE molecules on a mast cell → mast cell degranulation → histamine release → allergy/anaphylaxis
• Also important in defense against parasites (helminths/worms)
• Elevated in allergic diseases and parasitic infections

ANTIGEN TYPES: Thymus-Dependent vs Thymus-Independent

SUMMARY TABLE: Key Memory Pegs