Cells of the Immune System
There are three types of lymphocytes
- B cells
- T cells
- Natural killer cells
Lymphocytes trigger the adaptive immune response as they mediate the specific recognition of pathogens. Bone marrow stem cells give rise to all lymphocytes but the B and T cells mature in different places, T cells in the thymus and B cells in the bone marrow.
T cells have an antigen receptor (T cell antigen receptor) that binds antigens on the surface of other cells. There are also three main types of T cells, type 1 helper T cells which assist destruction of pathogens by phagocytes. The second kind of T cells are type 2 helper T cells, they interact with B cells and help them undergo clonal expansion and make their antibody. Cytotoxic T lymphocytes are the third kind, they are involved in immune responses triggered by viruses and they cause the death of infected host cells. T cells either act by cell-cell interactions or by the release of cytokines.
B cells express antibodies on their cell surface which are specific for a particular antigen. When they bind to their specific antigen the B cells divide and differentiate into plasma cells, the plasma cells produce and secrete the antibody, these bind to the same antigen that initially activated the B cells.
Natural killer cells are large granular lymphocytes are able to detect cells infected by a virus from changes on the cell surface. They destroy these cells but used a different method of detection to cytotoxic T lymphocytes.
There are three types of phagocytes involved in the immune response
- Mononuclear phagocytes
Mononuclear phagocytes are also known as monocytes, they can migrate to the site of infection. Once they move out of the blood stream and into the tissue they divide and differentiate into macrophages. These macrophages then engulf the pathogen and
present its antigens to nearby T cells.
Neutrophils also migrate into the tissue and engulf pathogens, however once they have phagocytosed the material they then die. Neutrophils make up the largest proportion of blood leukocytes and migrate mostly to sites of inflammation.
Eosinophils are less phagocytic than the other phagocytes, however they are able to attack extracellular pathogens and cause damage to them. They do this by degranulation, which releases cytotoxic cationic proteins that induce dysfunction in tissues that are close by. Eosinophils are particularly involved in hypersensitivity reactions such as allergic reactions.
Auxiliary cells mediate inflammation as part of the immune response. There are three main auxiliary cells involved in the immune response.
- Mast cells
Basophils are also a type of leukocyte along with the lymphocytes and phagocytes. They contain granules which on degranulation release histamine, and platelet activating factor. These inflammatory mediators cause increased vascular permeability and smooth muscle contraction. They are also able to synthesise and secrete other mediators that control the development of immune system reactions. Basophils are mobile cells that circulate in the blood stream.
Mast cells are similar to basophils, they also contain granules which are released when the cell is triggered and cause inflammation in surrounding tissue. However they are not circulating cells, they are found close to blood vessels in all types of tissue especially mucosal and epithelial tissues.
Platelets which normally function in blood clotting are also activated by an immune response and release inflammatory mediators.
Immunology, 7th edition, Male D. Brostoff J. Roth D. B. Roitt I. 2006, Elsevier.
Immunobiology: The Immune System in Health and Disease. 5th edition, Janeway C. A. Travers P. Walport M. 2001, Garland Science.
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