Memory tricks for immunity and fluid balance

Fluid balance: capillaries leak ~3 liters of fluid per day into tissues — lymphatic vessels collect it and return it to the blood. Without this, edema would be severe and fatal. Immunity: lymph is filtered through lymph nodes where pathogens are destroyed. Immune cells mature and circulate through lymphoid tissue. Dietary fat absorption: lacteals in the small intestine absorb dietary fats (chylomicrons) and deliver them to the blood via the thoracic duct — bypassing the portal system.

Primary lymphoid organs: where immune cells develop. Thymus: T-cell maturation and selection — most active before puberty, involutes with age. Red bone marrow: B-cell development. Secondary lymphoid organs: where immune responses occur. Lymph nodes: filter lymph, site of lymphocyte activation. Spleen: filters blood, largest lymphoid organ. MALT (mucosa-associated lymphoid tissue): tonsils, Peyer's patches, appendix — protects mucosal surfaces.

Innate immunity: non-specific, immediate (minutes to hours). Physical barriers (skin, mucus), phagocytes (neutrophils, macrophages), NK cells, complement, inflammation. No memory — same response every time. Adaptive immunity: specific, slow (days to weeks first time, faster second time due to memory). T cells and B cells. Has immunological memory — basis of vaccines. Both systems work together: innate activates adaptive, adaptive enhances innate. Think: innate = bouncers (stop everyone suspicious), adaptive = detectives (build case against specific targets).

Cytotoxic T cells (CD8+): kill infected cells, cancer cells, and transplanted cells directly. Helper T cells (CD4+): orchestrate the immune response — activate B cells, cytotoxic T cells, and macrophages. Regulatory T cells (Tregs): suppress immune response — prevent autoimmunity. Memory T cells: persist after infection — enable faster response on re-exposure. HIV destroys CD4+ helper T cells — that is why AIDS patients cannot mount immune responses. CD4 count below 200/μL = AIDS diagnosis.

B cells recognize antigens → activated by Helper T cells → differentiate into plasma cells → plasma cells secrete antibodies. Five antibody classes: IgG (most abundant, crosses placenta, secondary response), IgA (secretory, found in saliva/breast milk/mucus), IgM (first produced, largest, best at complement activation), IgE (allergies and parasites), IgD (B cell activation). Memory trick for classes: Go Away Monkeys Eating Dirt = G, A, M, E, D.

Lymph enters through afferent lymphatic vessels, percolates through the node, and exits via efferent vessels. Cortex (outer): contains lymphoid follicles with B cells — germinal centers form here during active immune response. Paracortex (middle): T cell zone — also contains dendritic cells presenting antigens. Medulla (inner): contains macrophages and plasma cells secreting antibodies. Lymph nodes enlarge (lymphadenopathy) during infection — the classic "swollen glands." Sentinel lymph node biopsy used in cancer staging.

The spleen is the largest lymphoid organ — located in the left upper quadrant, protected by ribs 9-11. Red pulp: filters blood, removes old/damaged RBCs and platelets, stores platelets (one-third of body's platelets). White pulp: lymphoid tissue surrounding arteries — immune response to blood-borne antigens. Fetal hematopoiesis: produces blood cells before birth (extramedullary hematopoiesis resumes in disease). Asplenic patients: at high risk for encapsulated bacteria (Strep pneumoniae, H. flu, Neisseria meningitidis).

Complement is a cascade of plasma proteins that enhance (complement) antibody function. Three activation pathways: Classical (triggered by antibody-antigen complex), Lectin (triggered by mannose on bacterial surfaces), Alternative (spontaneous activation on foreign surfaces). All three converge at C3 → C5 → MAC (Membrane Attack Complex) — literally punches holes in bacterial membranes. Other functions: opsonization (coats bacteria for phagocytosis), chemotaxis, inflammation. C3b is the key opsonin — coats bacteria for macrophages.

The thoracic duct is the largest lymphatic vessel — drains the entire body except the right upper quadrant (right side of head/neck, right arm, right thorax). It begins at the cisterna chyli (L1-L2), ascends through the thorax, and empties into the left subclavian vein. The right lymphatic duct drains only the right upper quadrant into the right subclavian vein. Chyle is the milky lymph from lacteals — contains dietary fat. Chylothorax = chyle leaks into chest cavity (thoracic duct injury).