adaptive immunity

Although innate immunity is critical for survival, over time pathogens have evolved mechanisms to evade and overcome innate immunity. It was under this selective pressure that an adaptive immune system evolved in vertebrates. The adaptive immune system first emerged in early fish and consists of lymphocytes and their products, which likely evolved from the more ancient innate immune system. As its name suggests, adaptive immunity is not preexistent to an encounter with a specific pathogen as is innate immunity. Instead, adaptive immunity develops with specificity for a pathogen when the body encounters that particular pathogen for the first time. Adaptive immunity is further distinguished from innate immunity because of a unique property: memory. That is, upon subsequent infection with the same pathogen, the adaptive immune system remembers the pathogen and mounts a faster and stronger response to eliminate the pathogen from the body. However, the adaptive immune system does not operate alone, but works in coordination with the innate immune system. The role of antibodies as opsonins directing phagocytosis has already been mentioned. Conversely, innate immune cells play roles in activating the adaptive immune system.
The cells of adaptive immunity are B and T lymphocytes. Lymphocytes make up 20-40% of the human leukocyte population and also include natural killer cells. B lymphocytes, or B cells, mature in the bone marrow. T lymphocytes, or T cells, mature in the thymus. B cells are responsible for the production of antibodies, which are the major effector molecules of humoral immunity. On the basis of their function in the immune response, T cells are divided into two categories: T helper (TH) lymphocytes and cytotoxic T lymphocytes (CTLs). TH cells orchestrate the activity of other cells of the immune system by releasing messenger molecules known as cytokines and are subdivided into T helper type 1 (TH1) and T helper type 2 (TH2) cells on the basis of the cytokines they produce. TH1 cytokines preferentially direct immune responses against pathogens that invade cells and against tumors, whereas TH2 cytokines preferentially direct immune responses against extracellular pathogens. CTLs eliminate target cells, such as tumor cells or cells infected by viruses, by producing molecules that form pores on their surface. CTLs then use the pores on the target to insert additional molecules that specifically induce cell death.

Source: http://www.creativebiomart.net/r ... ity-proteins_78.htm