This subcategory of Type IV hypersensitivity reaction is a delayed type hypersensitivity (DTH). Initial exposure to an antigen is known as the sensitization step and results in the formation of memory TH1 cells. Subsequent exposure these memory cells release cytokines that activate macrophages, which ultimately damage tissue.

Subcategory I type IV Hypersensitivity Reaction

This subcategory of Type IV hypersensitivity reaction involving CD4 TH2-mediated reactions causes either chronic asthma or chronic allergic rhinitis after the allergen is inhaled and eosinophils are recruited and activated.

Subcategory 2 Type IV Hypersensitivity Reaction

This type of Type IV hypersensitivity reaction is mediated by CK8 cytotoxic T cells and can result in contact dermatitis or tissue transplant rejection. Upon interaction with the antigen,  cytotoxic T cells are activated which induces granzyme-mediated apoptosis in cells presenting the antigen, including "self" cells.

Subcategory 3 Type IV Hypersensitivity Reaction

Type IV hypersensitivity reactions are further classified into subcategories based on T-Cell subtype, type of antigen, and the resulting effector mechanism.

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Type IV hypersensitivities occur when activated macrophages or cytotoxic T cells  lead to tissue damage. 

Type IV Hypersensitivity

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