Figure 1.

The different types of hypersensitivity responses based on the Gell and Coombs classification scheme. Type I hypersensitivity reactions are mediated by antigen-specific IgE molecules that facilitate degranulation of mast cells following antigen exposure. Type II hypersensitivity reactions are mediated by antigen-specific IgG/M molecules that recognize cell-associated antigens, causing destruction of the target cell (type IIa) or alterations in target cell functionality (type IIb). Type III hypersensitivity responses involve soluble antigen recognition by IgG/M molecules. This leads to the formation of antigen/immune complexes that can deposit in various tissues of the body, activate complement, and cause local tissue damage. Type IV hypersensitivity reactions involve the effector functions of various subsets of T-lymphocytes. Type IVa responses are mediated by CD4+ Th1 cells and result in activation of macrophages. Type IVb reactions involve the actions of CD4+ Th2 cells, which promote eosinophilic inflammation. Type IVc responses implicate CD8+ CTLs, which exert direct cytotoxic effects on target cells. Finally, type IVd reactions are mediated by various subsets of T-cells that facilitate the development of neutrophilic inflammation.