Learning Objective
Describe the four types of hypersensitivity reactions, including their mechanisms, mediators, clinical examples, and diagnostic tests.
Hypersensitivity Reactions
Four Types:
- Type I: Anaphylactic / Atopic – IgE-mediated
- Type II: Antibody-mediated – IgG or IgM against cell-surface or extracellular antigens
- Type III: Immune complex – IgG/IgM immune complexes
- Type IV: Cell-mediated (delayed-type) – T-cell mediated
Type I – Anaphylactic / Atopic (IgE-mediated)
| Feature | Details |
|---|---|
| Mechanism | Allergen crosslinks preformed IgE on mast cells → degranulation → histamine, tryptase, leukotrienes |
| Phases | Immediate (minutes): degranulation → vasodilation, edema, smooth muscle contraction Late (hours): chemokines recruit eosinophils → tissue damage |
| Tests | Skin prick test, allergen-specific IgE (ELISA) |
| Examples | Anaphylaxis (food, drugs, insect sting), allergic asthma |
| Mnemonic | “First and Fast” |
Type II – Antibody-mediated (Cytotoxic / Cell-surface antigen)
| Feature | Details |
|---|---|
| Mechanism | IgG/IgM antibodies bind cell-surface or ECM antigens → opsonization, complement activation, or antibody-dependent cellular cytotoxicity (NK cells) → cell destruction, inflammation, or receptor dysfunction |
| Tests | Direct Coombs = antibodies on RBCs Indirect Coombs = circulating antibodies in serum |
| Examples | Autoimmune hemolytic anemia, immune thrombocytopenia, transfusion reactions, hemolytic disease of newborn, Goodpasture syndrome, Rheumatic fever, Hyperacute transplant rejection, Myasthenia gravis, Graves disease, Pemphigus vulgaris |
| Mnemonic | “Second and Specific” (antibody specificity) |
Type III – Immune Complex–mediated
| Feature | Details |
|---|---|
| Mechanism | Antigen-antibody (mostly IgG) complexes → complement activation → neutrophil recruitment → tissue damage |
| Classic Forms | Serum sickness: fever, urticaria, arthralgia, proteinuria, lymphadenopathy 1–2 weeks post-antigen (foreign protein/drug) Arthus reaction: local edema/fibrinoid necrosis after intradermal antigen in a sensitized person |
| Examples | SLE, Polyarteritis nodosa, Poststreptococcal glomerulonephritis, IgA vasculitis, Reactive arthritis |
| Mnemonic | “Third is Complex” |
Type IV – Cell-mediated (Delayed-type)
| Feature | Details |
|---|---|
| Mechanism | T cells, no antibodies 1. CD8+ cytotoxic T cells → direct cell killing 2. CD4+ Th1 cells → cytokines → macrophage activation, inflammation |
| Tests | PPD (TB), patch test (contact dermatitis), Candida skin test (T-cell function) |
| Examples | Contact dermatitis (poison ivy, nickel), DRESS (drug reaction with eosinophilia and systemic symptoms), graft-versus-host disease |
| Mnemonic | “Fourth, T cells” → 4Ts: T cells, Transplant rejection, TB skin test, Touching (contact dermatitis)” |
You must be logged in to post a comment.