The primary agents of the immune response are lymphocytes, white blood cells (leukocytes) that originate in the bone marrow (like all blood cells) but concentrate in lymphoid tissues such as the lymph nodes, the thymus gland, and the spleen. When lymphocytes mature, they become immunocompetent, or capable of binding with a specific antigen. An immunocompetent lymphocyte displays unique proteins on its plasma membrane that act as antigen receptors. Because all of the antigen receptors of an individual lymphocyte are identical, only a specific antigen can bind to an individual lymphocyte. The kind of antigen receptors displayed by a particular lymphocyte is determined by somatic recombination, a shuffling of gene segments during lymphocyte maturation. By mixing gene segments, more than one billion different antigen receptors can be generated.

Here are the various kinds of lymphocytes:

  • B cells (B lymphocytes) are lymphocytes that originate and mature in the bone marrow. The antigen receptors of B cells bind to freely circulating antigens. When B cells encounter antigens that bind to their antigen binding sites, the B cells proliferate, producing two kinds of daughter cells, plasma cells and memory cells:

    • Plasma cells are daughter cells of B cells. Each plasma cell releases antibodies, proteins that have the same antigen binding capability as the antigen receptors of its parent B cell. Antibodies circulate through the body, binding to the specific antigens that stimulated the proliferation of plasma cells.

    • Memory B cells are long‐lived daughter cells of B cells that, like plasma cells, produce antibodies.However, memory cells do not release their antibodies in response to the immediate antigen invasion. Instead, the memory cells circulate in the body and respond quickly to eliminate any subsequent invasion by the same antigen. This mechanism provides immunity to many diseases after the first occurrence of the disease.

  • T cells (T lymphocytes) are lymphocytes that originate in the bone marrow, but mature in the thymus gland. The antigen receptors of T cells bind to self cells that display foreign antigens (with MHC proteins) on their plasma membrane. When T cells bind to these aberrant self cells, they divide and produce the following kinds of daughter cells:

    • Cytotoxic T cells (killer T cells) are activated when they recognize antigens that are mixed with the MHC‐I proteins of self cells. Following activation, cytotoxic cells proliferate and destroy the recognized cells by producing toxins that puncture them, thus causing them to lyse.

    • Helper T cells are activated when they recognize antigens that are mixed with the MHC‐II proteins of self cells. Proliferation produces helper T cells that intensify antibody production of B cells. Helper T cells also secrete hormones called cytokines that stimulate the proliferation of B cells and T cells.

    • Suppressor T cells are believed to be involved in winding down a successful immune response and in preventing the attachment of uninfected self cells.

    • Memory T cells are long‐lived cells possessing the same antigen receptors as their parent T cell. Like memory B cells, they provide a rapid defense to any subsequent invasion by the same antigen.