Interaction of microbial agents with the immune system during infectious disease

Research during the last years has revealed a considerable complexity of the immune system. It is clear that immunological reactions depend on extensive and only partly clarified interactions between a number of different cell types (e.g. B lymphocytes, plasma cells, T cell subpopulations, cytotoxic K and NK cells, monocytic cells, neutrophilic and eosinophilic granulocytes) and their molecular products (e.g. immunoglobulins, lymphokines and interleukins). These components further interact with the complement system, as well as with immunologically nonspecific components like acute phase proteins (e.g. C-reactive protein) and with other pathophysiological phenomena occurring during infections, e.g. the fever response. The application of these observations from basic and experimental immunology to the investigation of antimicrobial immune reactions is still only in its beginning, but has already resulted in new concepts of clinical value for the understanding of infectious diseases. The present paper briefly describes certain aspects of the immune response to infections with various microbial agents, with particular emphasis on reactions of clinical importance. In addition to B and T cell reactions, possible antimicrobial functions of K cells and NK cells are discussed, and the possible importance in infectious disease of various T cell subpopulations, particularly T suppressor cells, is discussed. Lastly, various escape mechanisms are mentioned whereby certain microbial agents may evade elimination by the immune response of the host.