The immune response is a complex series of cellular interactions activated by the entry into the body of foreign (nonself) antigenic materials such as infectious agents and a variety of macromolecules. After processing by macrophages, the antigen is presented to lymphocytes, which are the major effector cells of the immune system (Figure 4-1). Lymphocyte activation by antigen results in proliferation and transformation of the lymphocytes, which lead to two main types of immune response:
Summary of the immune response. Lymphocytes (both B and T) bearing specific antigen receptors are induced to proliferate (amplification phase) after they react with an antigen. The process by which lymphocytes recognize an antigen commonly involves an antigen-processing cell (various types of macrophages). Proliferation produces the effector cells of the immune response. Effector B cells (plasma cells) produce specific antibody, which mediates humoral immunity. Effector T cells exert a direct cytotoxic effect and mediate cellular immunity. Humoral immunity is so called because it can be transferred from an immune individual to a susceptible one by injection of serum containing antibody; cellular immunity can be transferred only by injection of live T cells. (MHC, major histocompatibility complex.)
Cell-mediated immunity is a function of T lymphocytes, leading to the production of effector (killer) T cells, which have the ability to destroy antigen-bearing cells by direct toxicity, and of specific products called lymphokines that mediate cell interactions (macrophage, T cell, B cell) in the immune response. Furthermore, two subtypes of T cells serve to modulate the immune response: helper T cells enhance it; suppressor T cells have the opposite effect.
Humoral immunity is a function of B cells and is characterized by the transformation of B cells into plasma cells, which secrete immunoglobulins (antibodies) that have specific activity against the inciting antigen.
The immune response is characterized by (1) specificity (ie, reactivity is directed toward and restricted to the inducing agent, termed the antigen); (2) amplification (the ability to develop an enhanced response on repeated exposure to the same antigen); and (3) memory (the ability to recognize and mount an enhanced response against the same antigen on subsequent exposure even if the first and subsequent exposures are widely separated in time). These features distinguish the immune response from other nonspecific host responses such as acute inflammation and nonimmune phagocytosis.
Tolerance to Self Antigens
The concepts of self and nonself (foreignness) are central to immunologic reactivity (Figure 4-2). Many molecules in a host individual are antigenic (ie, they induce an immune response) if introduced into another individual but are not recognized as antigens by the host. This failure to respond to self antigens is natural tolerance, and it prevents the immune system from destroying the host's own tissues. Tolerance to self antigens is induced during embryonic development, and it also demonstrates ...