Cells of the Immune System

CELLS OF THE IMMUNE SYSTEM

A schematic overview of the cells involved in both the innate and adaptive components of the immune response is shown in Figure 9.1. Most of the cells involved in the immune system arise from progenitor cell populations within the bone marrow. The differentiation of these progenitor cells is under the control of a variety of growth factors, e.g. granulocyte-or macrophage-colony stimulating factors (G-CSF and M-CSF, respectively) released by monocyte and macrophage cells as well as by fibroblasts and activated endothelial cells. These growth factors promote the growth and maturation of monocyte and granulocyte populations within the bone marrow before their release into the lymphoid and blood circulations.

The principal cells of the innate immune system include the following:

•         Mononuclear phagocytic cells, which are short-lived (< 8 hours) monocytes in the blood circulation that migrate into tissues and undergo further differentiation to give rise to the long-lived and key effector cell—the macrophage.

•         The granulocyte cell populations which include the neutrophil, basophil and eosinophil.

•         The mast cell which is a tissue-resident cell that is triggered by tissue damage or infection to release numerous initiating factors leading to an inflammatory response. Such initiating factors include histamine, leukotrienes B4, C4 and D4, proinflammatory cytokines (signal proteins released by leucocytes—white blood cells), such as tumour necrosis factor-α (TNF-α), and chemotactic substances such as interleukin-8 (IL-8). The sudden degranulation of the contents of mast cells is also responsible for the acute anaphylactic reactions to bee stings, penicillins, nuts, etc.

•         The natural killer (NK) cell which has a phenotype similar to that of lymphocytes but lacks their specific recognition receptors. The NK cell exploits non-specific recognition to elicit cytotoxic actions against host cells infected with virus and those host cells that have acquired tumour cell characteristics.

The lymphocyte populations also arise from bone marrow progenitor cells. The B-lymphocytes mature or differentiate in the bone marrow before leaving to circulate in the blood and lymph, while T-lymphocytes undergo maturation in the thymus. Antibodies mediating the effector functions of the humoral immune system are produced and secreted from a differentiated B-lymphocyte cell population termed plasma cells.