immune cells to an activated state that confers a short-term "immune memory". In this sense, "adaptive immunity" more closely resembles the concept of "activated state" or "heterostasis", thus returning in sense to the physiological sense of "adaptation" to environmental changes. Functions Acquired immunity is triggered in vertebrates when a pathogen evades the innate immune system and (1) generates a threshold level of antigen and (2) generates "stranger" or "danger" signals activating dendritic cells. The major functions of the acquired immune system include: Recognition of specific "non-self" antigens in the presence of "self", during the process of antigen presentation. Generation of responses that are tailored to maximally eliminate specific pathogens or pathogen-infected cells. Development of immunological memory, in which pathogens are "remembered" through memory B cells and memory T cells. In humans, it takes 4–7 days for the adaptive immune system to mount a significant response. Lymphocytes T and B lymphocytes are the cells of the adaptive immune system. The human body has about 2 trillion lymphocytes, which are 20–40% of white blood cells; their total mass is about the same as the brain or liver. The peripheral bloodstream contains only 2% of all circulating lymphocytes; the other 98% move within tissues and the lymphatic system, which includes the lymph nodes and spleen. In humans, approximately 1–2% of the lymphocyte pool recirculates each hour to increase the opportunity for the cells to encounter the specific pathogen and antigen that they react to. B cells and T cells are derived from the same multipotent hematopoietic stem cells, and look identical to one another until after they are activated. B cells play a large role in the humoral immune response, whereas T cells are intimately involved in cell-mediated immune responses. In all vertebrates except Agnatha, B cells and T cells are produced by stem cells in the bone marrow. T cell progenitors then migrate from the bone marrow to the thymus, where they develop further. In an adult animal, the peripheral lymphoid organs contain a mixture of B and T cells in at least three stages of differentiation: Naive B and naive T cells, which have left the bone marrow or thymus and entered the lymphatic system, but have yet to encounter their matching antigen Effector cells that have been activated by their matching antigen, and are actively involved in eliminating a pathogen Memory cells, the survivors of past infections Antigen presentation Acquired