The document classifies and describes the four main types of hypersensitivity reactions: 1. Type I involves IgE antibodies, mast cells, and TH2 cells causing immediate hypersensitivity reactions. 2. Type II involves IgG and IgM antibodies injuring cells through phagocytosis or lysis and inducing inflammation. 3. Type III involves IgG and IgM antibodies forming complexes with antigens in circulation that deposit in tissues, inducing inflammation through recruitment of neutrophils and monocytes. 4. Type IV involves sensitized T lymphocytes like TH1, TH17 cells and CTLs causing tissue injury in cell-mediated immune disorders.