Basic Components of the Immune System

This chapter is not a comprehensive review of immunology but rather a condensed version of those aspects of immunology that have particular relevance to clinical immunology. Refer to the Bibliography for a more extensive discussion of the role of each component. It is generally believed that the immune system evolved as the host’s defense against infectious agents, and it is well known that patients with defi ciencies in the immune system generally succumb to these infectious diseases. However, as we shall see, it may well play a larger role in the elimination of other foreign substances, including tumor antigens or cells and antibodies that attack self. An immune response may be conveniently divided into two parts: (1) a specifi c response to a given antigen and (2) a more nonspecifi c augmentation to that response. An important feature of the specific response is that there is a quicker response to the antigen during a second exposure to that antigen. It is the memory of the initial response that provides the booster effect. For convenience, the specifi c immune response may be divided into two parts: (1) the humoral response and (2) the cellular response to a given antigen. As we shall see, however, both responses are mediated through the lymphocyte. Humoral responses are antibodies produced in response to a given antigen, and these antibodies are proteins, have similar structures, and can be divided into various classes of immunoglobulins. Cellular responses are established by cells and can only be transferred by cells. (See the Bibliography for the extraordinary beginnings of the concept of a cellular arm of the immune system.) Up to the 1940s the general dogma held that only antibodies were involved in the immune response. Dr. Merrill Chase, who began his experiments in a laboratory devoted primarily to the humoral response, clearly showed in a series of elegant experiments that immunity was not just humoral but that a cellular response by the lymphocytes could also produce immunity. Some of the best examples of the power of cellular immunity may be found in the many experiments in which transfer of cells can induce autoimmune disease in animals and humans as well as rejection of an organ graft in both animals and humans by cells. The separation of human and cellular immunity was further advanced by the study of immunodefi cient humans and animals. For example, thymectomized or congenitally athymic animals as well as humans cannot carry out graft rejection, yet they are capable of producing some antibody responses. The reverse is also true. Children (and animals) who have an immune defi cit in the humoral response do not make antibodies but can reject 1. Basic Components of the Immune System