Cellular and genetic requirements for delayed type hypersensitivity

All living organisms are continuously exposed to pathogenic agents from the surrounding milieu, e.g. viruses, bacteria, fungi and parasites. Skin and mucous membranes form efficient barriers to these agents, but sometimes this defense is overcome. If an infectious agent succeeds in penetrating into the "milieu int€rieur", at first granulocytes and mononuclear phagocytes become involved in the elimination of the intruding microorganisms. This part of the defense system is largely aspecific. Apart from this, vertebrates have a well developed immune system which can mount specific immune responses to invading organisms and foreign substances. The specificity of the immune reaction is based on the presence of receptors on the individual lymphocytes which recognize specifically any one of many foreign substances. Only after the recognition of the immunogenic material is a lymphocyte activated to perform its function. Immunogenic substances or antigens are operationally defined by their capacity to induce an immune response. The recognition of a particular antigen by the receptors of an individual lymphocyte is predetermined, i.e. the diversity of lymphocytes, each specific for one of the numerous imaginable foreign substances is generated before encounter with immunogenic material. In addition to the specificity of lymphocytes for a particular antigen, lymphocytes must also discriminate between self and non-self. If they should fail to do so, an immune response to tissues of the individual's own body would arise, leading to autoimmune disease (Burnet, 1972).