Lymphocytes are the white blood cells (leukocytes) which are directly involved in the immune response to an antigen that has entered the body; an antigen can be a bacterium and its toxin, a virus, or a parasite. They originate in the bone marrow from undifferentiated precursor cells, and then they develop and mature in the spleen and thymus into B-lymphocytes and T-lymphocytes, also known as B cells and T cells. They are found in the bloodstream, lymphatic nodes and vessels, as well as in the lymphoid tissue of thymus and spleen.
B lymphocytes (B cells)- They leave the bone marrow as progenitor (precursor) cells; then they defuse throughout the body, developing into differentiated (mature) B cells in the white pulp of spleen. About 30% of lymphocytes in the body are B cells, which immediately transform into plasma cells when they detect the presence of an antigen. The plasma cells in turn secretes antibodies called immunoglobulins. Immunoglobulins adhere to antigens, marking them, so that the killer T cells can identify and destroy them. This creates what is called immunological memory.
T lymphocytes (T cells)- They comprise about 70% of circulating lymphocytes. They also arise in the bone marrow but they mature in the thymus as immunologically efficient cells. During antigenic stimulation, they undergo clonal proliferation to produce killer T cells (or toxic T cells), helper T cells, suppressor T cells, and lymphokines. Killer T cells mediate the lysis (destruction) of foreign pathogens or abnormal cells, which have been marked by immunoglobulins. Helper T cells stimulate B cell differentiation (tranformation) into plasma cell, or immunoglobulin.
Below, a schematic picture which shows how a B lymphocyte develops into a plasma cell, which produce antibodies.
Antibodies produce immunoglobulins
Below, the picture shows how an immature T cell can develop into either a helper T cell or killer T cell.
