One of the two major classes of lymphocytes, B lymphocytes are blood cells of the immune system, derived from the bone marrow and spleen, and involved in the production of antibodies. During infections, these cells are transformed into plasma cells that produce large quantities of antibody directed at specific pathogens. When antibodies bind to foreign proteins, such as those that occur naturally on the surfaces of bacteria, they mark the foreign cells for consumption by other cells of the immune system. This transformation occurs through interactions with various types of T cells and other components of the immune system. In persons living with AIDS, the functional ability of both the B and the T lymphocytes is damaged, with the T lymphocytes being the principal site of infection by HIV.