| GELL AND COOMBS CLASSIFICATION OF HYPERSENSITIVE REACTIONS | ||||
| TYPE | DESCRIPTIVE NAME |
INITIATION TIME |
MECHANISM | TYPICAL |
| IMMEDIATE REACTIONS | ||||
| Type I | IgE-mediated hypersensitivity |
2-30 min | Ag induces cross-linkage of IgE bound to mast cells and basophils with release of vasoactive mediators | Systematic anaphylaxis Localized anaphylaxis: - Hay fever - Asthma - Hives - Food allergies - Eczema |
| Type II | Antibody- mediated cytotoxic hypersensitivity |
5-8 h | Ab directed against cell-surface antigens mediates cell destruction via complement activation or ADCC | - Blood-transfusion reactions - Erythroblastosis fetalis - Autoimmune hemolytic anemia |
| Type III | Immune complex- mediated hypersensitivity |
2-8 h | Ag-Ab complexes deposited in various tissues induce complement activation and an ensuing inflammatory response | - Localized Arthus reaction Generalized reactions: - Serum sickness - Glomulonephritis - Rheumatoid arthritis - Systemic lupus erythematosus |
| DELAYED REACTIONS | ||||
| Type IV | Cell-mediated hypersensitivity |
24-72 h | Sensitized TDTH cells release cytokines that activate macrophages or TC cells, which mediate direct cellular damage | - Contact dermatitis - Tubercular lesions - Graft rejection |