ABSTRACT
Differing defence mechanisms are required to deal with infectious agents that occupy different compartments within the body. The immune system has evolved for protection against infectious agents that invade the body. Inflammatory processes are necessary to recruit components of the immune system to sites of infection. The immune system has evolved regulatory processes to limit the latter but, in some instances, immunological tissue damage becomes the main clinical feature, generating the problems of hypersensitivity. The innate and adaptive systems interact and synergize to generate optimal immunity. Lymphocyte activation is dependent not only on antigen recognition, but also on co-stimulatory signals provided by cells and molecules of the innate system. The antibodies produced by B cells and the cytokines secreted primarily by T cells enhance the defensive activities of the innate system. Antibodies play a major role in the adaptive immune response to extra-cellular pathogens. In the environment the microbes are protected from the activities of extra-cellular antibodies and complement proteins.
