PA8.1-2 | Immunity & Hypersensitivity Reactions — Summary & Reflection

REFLECT

Consider three patients on the same medical ward:

1. Patient A — 18-year-old with bee-sting anaphylaxis, managed in emergency
2. Patient B — 45-year-old with Goodpasture syndrome (pulmonary haemorrhage + renal failure)
3. Patient C — 32-year-old with SLE and an acute nephritis flare; serum C3 is low

For each patient:
• Which Gell & Coombs type is operative?
• What is the specific effector mechanism causing tissue damage?
• Which investigation would confirm the immune mechanism? (think: serum C3/C4, immunofluorescence pattern, serology, skin test, skin biopsy)
• If the three patients were given the same immunosuppressant (high-dose corticosteroids), which patient would you expect to respond most durably, and why?

Jot your answers — then compare with the faculty notes at the end of this module.

KEY TAKEAWAYS

Immunity — two arms, one goal:
• Innate: immediate, non-specific — barriers, phagocytes, NK cells, complement
• Adaptive: delayed, specific, memory — humoral (B/IgG) and cell-mediated (CD4/CD8 T cells)
• CD4 Th subsets: Th1 (IFN-γ, intracellular pathogens), Th2 (IL-4/5/13, allergy), Th17 (IL-17, extracellular fungi), Treg (IL-10, tolerance)
• MHC I → endogenous peptides → CD8⁺; MHC II → exogenous peptides → CD4⁺

Hypersensitivity — Gell & Coombs four types:

• Type I (Immediate): IgE + mast cells → histamine (early) + leukotrienes (late) → anaphylaxis, asthma, atopy
• Type II (Cytotoxic): IgG/IgM on cell surface → opsonization/lysis/receptor dysfunction → AIHA, Goodpasture, MG, Graves, transfusion reactions
• Type III (Immune complex): Circulating IC deposit → complement → neutrophilic vasculitis → SLE, serum sickness, post-strep GN
• Type IV (Delayed): T cells → Th1/macrophage activation or CTL killing → TB granuloma, contact dermatitis, transplant rejection, type 1 DM

High-yield discriminators:
• Timing: minutes = Type I; hours = Types II/III; 48–72 h = Type IV
• Complement low in serum = Type III (consumed); complement normal = Types I, II-receptor, IV
• Granular IF = Type III; linear IF = Goodpasture (Type II)
• Antibody-transferable in serum = Types I–III; transfer requires T cells = Type IV