Cells of the Immune System

Abstract

Abstract The immune system includes a range of cell types with different roles in defending the body against infection. Many of these cells arise in the bone marrow, circulate in the blood and can migrate into solid tissues. Immune responses involve interactions between some of these cells and/or their secreted products. B and T lymphocytes specifically react to microbial antigens: activated B lymphocytes secrete antigen‐binding antibodies, and subpopulations of T lymphocytes possess regulatory or cytotoxic functions. Natural killer cells are also cytotoxic cells of the lymphoid lineage, but they do not possess properties of antigen recognition. Different types of dendritic cells present antigens to T and B lymphocytes. Blood monocytes give rise to tissue macrophages that are phagocytes, as are circulating neutrophils, which are the most plentiful type of granulocyte. Other circulating granulocytes are eosinophils that secrete toxic mediators, and basophils that, in common with tissue mast cells, are important sources of inflammatory mediators. Other cells contribute to immune and inflammatory responses, including endothelial cells, erythrocytes and platelets. Key Concepts: A variety of cell types are important components of the immune system. B and T lymphocytes specifically recognise antigens and are responsible for adaptive (acquired) immunity. B lymphocytes recognise native (unprocessed) antigens via surface immunoglobulins and produce secreted immunoglobulins (antibodies). T lymphocytes recognise processed antigens (usually peptides) associated with MHC proteins expressed on the surface of antigen‐presenting cells. Different T lymphocyte subpopulations have helper, cytotoxic or regulatory functions. Natural killer (NK) cells are lymphoid cells that lack antigen‐specific receptors, but mediate cytotoxic activity against infected or malignant cells. Dendritic cells are potent antigen‐presenting cells involved in the activation of T lymphocytes, whereas follicular dendritic cells present antibody‐associated antigens to B lymphocytes. Monocytes are circulating blood cells that give rise to tissue macrophages with phagocytic and antigen presentation functions. Granulocytes circulate in the blood, migrate into tissues, and include phagocytic neutrophils and eosinophils that secrete toxic mediators, and basophils that release inflammatory mediators; mast cells are tissue cells with similar properties to basophils. A variety of other cell types contribute to the generation and regulation of immune and inflammatory responses, including endothelial cells, erythrocytes and platelets.