<Li> Generation of responses that are tailored to maximally eliminate specific pathogens or pathogen - infected cells . </Li> <Li> Development of immunological memory, in which pathogens are "remembered" through memory B cells and memory T cells . </Li> <P> The cells of the adaptive immune system are T and B lymphocytes; lymphocytes are a subset of leukocyte . B cells and T cells are the major types of lymphocytes . The human body has about 2 trillion lymphocytes, constituting 20--40% of white blood cells (WBCs); their total mass is about the same as the brain or liver . The peripheral blood contains 2% of circulating lymphocytes; the rest move within the tissues and lymphatic system . </P> <P> B cells and T cells are derived from the same multipotent hematopoietic stem cells, and are morphologically indistinguishable from 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 . </P>

Where are the cells of the adaptive immune system found