The organs of the immune system are located throughout the body and include the bone marrow, spleen, thymus, tonsils, adenoids, appendix, and lymph nodes. These organs are involved in the production, development, or dissemination of lymphocytes, one of five types of white blood cells that circulate throughout the body. The role of lymphocytes is to recognize and fight infection and disease.

T Cells and B Cells

The two major classes of lymphocytes are T cells and B cells. They are both produced in the bone marrow and are named for the organ in which they mature.

• B cells mature in the bone marrow. The main role of B cells is to produce and secrete antibodies, special proteins that recognize and bind to antigens in order to inactivate or destroy them. Antibodies belong to a large family of proteins known as immunoglobulins. There are many types of antibodies, each of which acts on its corresponding antigen. Each B cell is programmed to make one specific antibody. For example, one B cell may make an antibody for a pneumonia bacterium, while another B cell may make an antibody for a cold virus or a marker of the melanoma cell.
• T cells mature in the thymus gland. Two types of T cells play a vital role in the immune response:
  • Cytotoxic T (CD8) cells kill cancer cells and virus-infected cells.
  • Helper T (CD4) cells activate other immune cells, including B cells and cytotoxic T cells. Precursors of mature helper T cells include:
    • T_H1, which produces cytokines, proteins that stimulate the activity of immune cells, including cytotoxic T cells. Examples of cytokines include interferons and interleukins.
    • T_H2, which helps prime B cells for antibody production.