Hypersensitivity : Immunological

The immune response activatesB cells to produce antibody and T cells to mediate cytotoxicity or produce cytokines. An important division in the development of T cells is their tendency to develop into subpopulations producing either TH1 or TH2 cytokines. The TH1 cytokines include interferon g, tumour necrosis factor b (TNFb) and interleukin 2 (IL-2), while the cytokines which best characterize the TH2 subset are IL-4, IL-5, IL-9, IL-10 and IL-13. T cells also produce other cytokines which are found in both pathways, and some which are produced by other types of cell. The TH1 pathway is stimulated by IL12, which is produced by antigen-presenting cells, and by interferon g, which activates antigen presenting cells. IL-4 enhances the development of the TH2 pathway by inhibiting the production of one of the chains of the IL12 receptor. This feedback tends to polarize cells to one or other type of response in the effector phase of the immunity. The TH2 cytokines help immunoglobulin E (IgE) and IgG4 (IgG1 in the mouse) antibody production and stimulate eosinophilia and the survival of eosinophils in tissues, while the TH1 cytokines activate macrophages and provide help for the other classes of IgG antibody. Cytokines, especially TNFa produced by T cells and by macrophages, can activate the production of chemokines which have chemotactic properties for different types of inflammatory cells. The interaction of all these factors, as well as the complement system, determine the effector mechanisms of the immune response. These can be protective or may result in tissue damage, which is called hypersensitivity.