The immune system in humans allows the body to fight disease by producing antibodies that are able to deactivate invading molecules, usually in the form of proteins or polysaccharides, called antigens. The cells that produce the antibodies are called lymphocytes, a type of leukocyte found in blood, in lymph, and in the lymphoid tissue of the spleen, thymus, and lymph nodes. Lymphocytes arise from lymphoid stem cells which, in turn, arise from hematopoietic stem cells, the cells that produce all leukocytes as well as erythrocytes. Before antigen stimulation, lymphocytes are small, metabolically inert cells that can circulate freely between lymph and blood vessels by moving through the gaps in the endothelial cells of the lymph and blood capillary walls. When stimulated by an antigen, the small, inactive lymphocyte grows and begins cell division producing plasma cells and memory cells. Plasma cells secrete the antibodies that deactivate the invading antigens. Memory cells resemble exactly the original lymphocyte and, thus, make it possible for the body to carry out a more rapid response to subsequent invasions by the same type of antigen since the multiple copies of the often times single original lymphocyte capable of responding to the particular antigen will remain in circulation.
B lymphocytes, which arise from those lymphoid stem cells in red bone marrow, secrete antibodies that circulate freely in solution. B lymphocyte antibodies usually attack antigens which are also free in solution, thus, providing the body with a humoral immunity. Because each antibody molecule can bind two antigen
