Adaptive or Acquired Immunity

Adaptive imm5ne responses are more sophisticated than innate immune responseq. Unlike innate immunity adaptive immunity is also highly specific to the pathogen.  The adaptive immune system can also remember prior experiences. This is why we develop lifelong immunity to many common infectious diseases after our initial exposure to the pathogen. If an animal is immunized with an antigen, an immune response (either antibody or cell mediated) appears after several days, rises rapidly and then more graudally declines in what is called a primary immune response. If later on the same animal is reinjected with the same antigen, it will typically produce a secondary immune response where the response to the antigen is quicker and greater. This is because when naive cells encountered antigen for the first time, some of them were stimulated to proliferate and differentiate into effector cells but some of these nave cells were also stimulated to multiply and differentiate into memory cells. The membory cells are not engaged in a response but are more easily induced to become effector cells by a later encounter with the same antigen.

Adaptive immune responses are carried out by white blood cells called lymphocytes. There are two broad classes of such adaptive responses; (1) antibody or humoral responses which are carried out b B cells and (2) cellular immune responses which are carried out by T cells.

Humoral Immune Response

The humoral immune response is uniquely adapted to the elimination of extracellular pathogens. It is characterized by the production of large numbers of antibody molecules specific for antigenic determinatns (epitopes) on the foreign pathogen. In the humoral response, antigen induces the clonal proliferation of B lymphocytes into antibody secreting plasma cells and B memory cells.

Cell-Mediated Immune Responses

Unlike the humoral branch of the immune system which serves mainly to eliminate extracellular bacteria, the cell mediated branch bears intracellular pathogens, virus infected cells, tumor cells and foreign grafts. The system is adapted to recognizing altered self cells and eliminating them. In the cellular branch, the recognition of an antigen MHC complex by a specific T lymphocyte induces clonal proliferation into various T cells with effector functions, such as T_H and CTLs, and T memory cells.

Both antigen specific and nonspecific cells contribute to the cell mediated immune response. Specific cells include CD4 T lymphocytes and CD8 T lymphoctyes. Nonspecific cells include macrophages, neutrophils, easinophils, and natural killer cells.