Adaptive Immune System - Effector Cells

Effector Cells

The cells of the adaptive immune system are a type of leukocyte, called a lymphocyte. B cells and T cells are the major types of lymphocytes. The human body has about 2 trillion lymphocytes, constituting 20-40% of white blood cells (WBCs); their total mass is about the same as the brain or liver. The peripheral blood contains 20–50% of circulating lymphocytes; the rest move within the lymphatic system.

B cells and T cells are derived from the same multipotent hematopoietic stem cells, and are morphologically indistinguishable from one another until after they are activated. B cells play a large role in the humoral immune response, whereas T-cells are intimately involved in cell-mediated immune responses. However, in nearly all other vertebrates, B cells (and T-cells) are produced by stem cells in the bone marrow. T-cells travel to and develop in the thymus, from which they derive their name. In humans, approximately 1-2% of the lymphocyte pool recirculates each hour to optimize the opportunities for antigen-specific lymphocytes to find their specific antigen within the secondary lymphoid tissues.

In an adult animal, the peripheral lymphoid organs contain a mixture of B and T cells in at least three stages of differentiation:

• naive cells that have matured, left the bone marrow or thymus, have entered the lymphatic system, but that have yet to encounter their cognate antigen,
• effector cells that have been activated by their cognate antigen, and are actively involved in eliminating a pathogen.
• memory cells – the long-lived survivors of past infections.