Memory tricks for fluid balance and the body's defenses
Lymphatic vessels, lymph nodes, lymphoid organs, innate and adaptive immunity, T cells, B cells, antibodies, and hypersensitivity — this section ties together the anatomy of the lymphatic system with the physiology of how the immune system protects the body.
Proven Mnemonics & Acronyms — fast to learn, hard to forget.
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Lymphatic System Functions
FIF — Fluid balance · Immunity · Fat absorption
Three core functions — return excess fluid, defend against pathogens, absorb dietary fat
Three functions of the lymphatic system — fluid, immunity, and fat
Fluid balance: capillary filtration produces ~3 L/day excess interstitial fluid — lymphatic capillaries absorb this and return it to blood via thoracic duct (left side) and right lymphatic duct (right side). Without lymphatics → edema. Immunity: lymph passes through lymph nodes where pathogens, cancer cells, and debris are filtered and destroyed by macrophages and lymphocytes. Lymphoid organs (thymus, spleen, lymph nodes, MALT) are sites of immune cell development and activation. Fat absorption: lacteals (lymphatic capillaries in intestinal villi) absorb dietary fats (chylomicrons) → chyle → thoracic duct → blood. Bypasses portal system.
Thoracic duct
Largest lymphatic vessel — drains left side of body and both legs → left subclavian vein.
Right lymphatic duct
Drains right arm, right side of head and thorax → right subclavian vein.
Primary lymphoid organs (T and B cell development) · Secondary (immune responses)
Four lymphoid organs — where immune cells mature and where they act
Thymus: primary lymphoid organ — where T cells mature and learn self-tolerance. Located in mediastinum. Most active in childhood — involutes after puberty. Cortex (immature T cells) → medulla (mature T cells). Thymic selection: positive (T cells that recognize self-MHC survive) + negative (T cells that attack self-antigens eliminated). Spleen: largest lymphoid organ — filters BLOOD (not lymph). White pulp (immune responses to blood-borne antigens) and red pulp (destroys old RBCs). Lymph nodes: filter LYMPH — stationed along lymphatic vessels. Site of adaptive immune responses to antigens. Cortex (B cells) + paracortex (T cells) + medulla. MALT (Mucosa-Associated Lymphoid Tissue): tonsils, Peyer's patches (ileum), appendix — defend mucosal surfaces.
Thymus
T cell school — positive and negative selection. Primary organ. Atrophies with age.
Spleen
Filters blood. Removes old RBCs. Largest lymphoid organ. Stores platelets.
Lymph nodes
Filter lymph. Swell during infection (lymphadenopathy). B cells in cortex, T cells in paracortex.
MALT
Tonsils, Peyer's patches, appendix. IgA production — secretory immunity at mucosal surfaces.
MHC I × CD8 = 8 · MHC II × CD4 = 8 · Two signals to activate
MHC I → CD8+ cytotoxic · MHC II → CD4+ helper · Signal 1 + Signal 2 required
How T cells recognize antigens — MHC restriction and the two-signal rule
MHC class I: on ALL nucleated cells — presents intracellular peptides (virus, tumor) to CD8+ cytotoxic T cells → cell killed. MHC class II: on professional APCs only (dendritic cells, macrophages, B cells) — presents extracellular peptides to CD4+ helper T cells → activates response. Memory trick: MHC I × CD8 = 8. MHC II × CD4 = 8. Both products equal 8. T cell activation requires TWO signals: Signal 1 = TCR binds MHC-peptide. Signal 2 (costimulation) = CD28 on T cell binds B7 on APC. Signal 1 alone → anergy (unresponsiveness). This two-signal requirement prevents autoimmune reactions to self-antigens. CTLA-4 competes with CD28 — immune checkpoint exploited in cancer immunotherapy.
MHC I + CD8
Intracellular antigens (virus, tumor). Cytotoxic T cell kills the infected cell.
MHC II + CD4
Extracellular antigens. Helper T cells activate macrophages or help B cells make antibodies.
Signal 2
CD28 + B7 costimulation. Without it → anergy. APCs upregulate B7 during infection.
CTLA-4 / PD-1
Immune checkpoints — dampen T cell response. Cancer immunotherapy blocks these to unleash T cells.
Five antibody classes — where each is found and what each does
B cells are activated by antigen + T cell help → proliferate in germinal centers → affinity maturation (somatic hypermutation) → class switching → plasma cells (antibody factories) or memory B cells. IgM: first antibody produced — pentamer, excellent at complement activation, cannot cross placenta. Primary response marker. IgG: most abundant (80%) — crosses placenta (maternal immunity), secondary response, opsonization, complement. IgA: secretory — in tears, saliva, breast milk, GI/respiratory mucus — first line at mucosal surfaces. IgE: lowest concentration but highest potency for allergy — binds mast cells and basophils → degranulation → anaphylaxis. IgD: mainly on naive B cell surface as BCR. Allergic response: IgE on mast cells + allergen → histamine → urticaria, anaphylaxis.
IgM
First produced (primary response). Pentamer. Strong complement activator. Cannot cross placenta.
IgG
Most abundant. Crosses placenta → newborn immunity. Secondary response. Opsonization.
IgA
Mucosal surfaces. Dimer in secretions. Breast milk → passive immunity for newborn.
IgE
Allergy and parasites. Binds mast cells. Anaphylaxis treated with epinephrine.
Active = you make it · Passive = you borrow it · Natural vs Artificial
Active (long-lasting with memory) · Passive (immediate but temporary, no memory)
Four types of immunity — how each is acquired and how long it lasts
Active immunity: body makes its own antibodies and memory cells → long-lasting. Natural active: infection → recovery → lifelong immunity (chickenpox). Artificial active: vaccination → immunological memory without disease. Passive immunity: pre-made antibodies transferred → immediate protection but no memory → temporary (weeks to months). Natural passive: IgG crosses placenta + IgA in breast milk → newborn protection for 3-6 months. Artificial passive: injection of immune serum (immunoglobulins) → used for rabies exposure, snakebite, tetanus post-exposure. Herd immunity: when enough of population is immune (natural or vaccine) → pathogen cannot spread → even unvaccinated are protected. Threshold varies (measles ~95%, polio ~85%).
Natural active
Infection → own immune response → memory. Often lifelong. Some pathogens evade (HIV, flu).
Artificial active
Vaccination. Attenuated, killed, subunit, mRNA, or toxoid vaccines.
Natural passive
IgG via placenta + IgA via breast milk. Protects newborn 3-6 months.
Artificial passive
IVIG, rabies immune globulin, antivenom. Immediate but no memory. Temporary.
Q: What are the primary and secondary lymphoid organs and their functions?
A: Primary lymphoid organs (where lymphocytes develop): Bone marrow — B cell development and maturation; T cell precursors originate here. Thymus — T cell maturation; positive selection (must recognize MHC) and negative selection (must not react to self-antigens → tolerance). Thymus involutes after puberty. Secondary lymphoid organs (where immune responses occur): Lymph nodes — filter lymph, site of B and T cell activation and clonal expansion. Spleen — filters blood, responds to blood-borne antigens, removes encapsulated bacteria (asplenic patients at risk for Strep pneumo, H. flu, Neisseria). MALT (mucosa-associated lymphoid tissue) — Peyer patches in intestine, tonsils — respond to mucosal pathogens.
Q: What is MHC class I vs class II and which T cells recognize each?
A: MHC class I (HLA-A, B, C): expressed on ALL nucleated cells. Presents endogenous antigens (intracellular proteins — viral peptides, tumor antigens) to CD8+ cytotoxic T cells. Rule: 8 × 1 = 8 (CD8 recognizes MHC I). MHC class II (HLA-DR, DP, DQ): expressed only on professional APCs (dendritic cells, macrophages, B cells). Presents exogenous antigens (phagocytosed pathogens) to CD4+ helper T cells. Rule: 4 × 2 = 8 (CD4 recognizes MHC II). Practical significance: organ transplant rejection — MHC mismatch triggers immune response. HLA typing before transplant. HLA associations: HLA-B27 → ankylosing spondylitis, reactive arthritis, psoriatic arthritis. HLA-DR4 → rheumatoid arthritis. HLA-DQ2/DQ8 → celiac disease.
Q: Describe B cell activation and the five classes of antibodies.
A: B cell activation: BCR binds antigen → internalized and presented on MHC II → CD4+ Th2 cell provides help via CD40L-CD40 interaction and IL-4 → B cell proliferates and differentiates → plasma cells (secrete antibody) and memory B cells. Class switching: from IgM → IgG, IgA, or IgE depending on cytokine environment. Five classes: IgM (pentamer, first response, best complement activator, agglutination), IgG (most abundant, crosses placenta, secondary response, opsonization), IgA (dimer with secretory component, found in secretions — breast milk, saliva, tears, gut), IgE (mast cell binding, parasites, allergy), IgD (B cell surface receptor, function unclear). Opsonization: IgG and C3b coat bacteria → phagocytes with Fc and complement receptors engulf more efficiently.
Q: What is the complement system's role and what are the clinical consequences of deficiencies?
A: Three activation pathways → converge at C3: Classical (antibody-dependent), Lectin (MBL binds mannose), Alternative (spontaneous C3 hydrolysis on pathogen surfaces). Effects: C3b opsonization (phagocytosis), C3a/C5a anaphylatoxins (inflammation, mast cell degranulation), MAC (C5b-9) kills Gram-negative bacteria. Clinical deficiencies: C1q deficiency → impaired immune complex clearance → SLE-like disease. C3 deficiency → most severe → recurrent encapsulated bacterial infections (pneumococcus, H. flu, meningococcus). C5-C9 (MAC) deficiency → recurrent Neisseria infections (need MAC to lyse them). DAF (CD55/59) deficiency → PNH (paroxysmal nocturnal hemoglobinuria) — RBCs lyse without protection from self-complement.
Q: What is the difference between primary and secondary immune responses?
A: Primary response (first antigen exposure): lag period 5-7 days, peak antibody response after 10-14 days. Predominantly IgM initially, then class switching to IgG. Lower titer, shorter duration. Memory B and T cells formed. Secondary response (re-exposure): lag period 1-3 days, much higher antibody titer (10-100x), predominantly IgG (already class-switched), longer duration. Basis of vaccination — prime with antigen → memory cells → booster doses amplify memory → protective titers achieved. Passive transfer: can give preformed antibodies for immediate protection without waiting for primary response (used in post-exposure prophylaxis). Affinity maturation: during primary response, B cells with higher affinity BCRs are selectively expanded in germinal centers → antibodies get better at binding over time.