Structure of antibodies
|Molecule of the Month|
Antibodies are Y-shaped proteins secreted by plasma cells (B-cells after exposure to an antigen) that are designed to bind to antigens, this in turn inhibits the function of said antigen by modifying its structure, that is, if this antigen is important to the function of the invader[note 1] and/or it invokes a antibody-dependent cell-mediated cytotoxicity (ADCC; ADCC only occurs with IgG and IgE-coated cells) and/or it causes agglutination[note 2] and/or they precipitate[note 3] which all (except, of course, ADCC) trigger the phagocytosis of the antibody/antigen complex (or the cell on which they are found). Alternatively if multiple antibodies bind to antigens on one cell it can trigger complement-mediated cell lysis.[note 4]
There are five major subtypes (or isotypes) of antibody (which can have multiple 'subunits' (each composed of a 'Y' structure)) which are displayed in the following table:
|IgA||Readily crosses the endothelium; neutralizes antigens. Found in almost all bodily fluids and helps to prevent pathogens from binding to mucus membranes and skin. Comes in monomeric (one subunit; mostly found in the plasma in small amounts) and dimeric (two subunits) forms.|
|IgD||Serves (along with monomeric IgM) as a B-cell antigen receptor. It is solely monomeric.|
|IgE||Sensitizes mast cells to release chemical mediators like those involved in allergic reactions (e.g., histamine). Plays a role in eosinophil-mediated ADCC against parasites. Stem end binds to basophils or mast cells; these cells become activated when the antigen binds to the receptor site of the IgE. Comes solely as a monomeric form and is found in the skin and mucosa of the digestive/respiratory tracts and tonsils, although traces are also found in blood plasma.|
|IgG||Exists solely as a monomeric form; found in abundance in the plasma, where it accounts for 75-85% of circulating antibodies, also crosses the placenta where it provides immunity for the fetus against all infections (and their associated toxins, if they are caused by bacteria) the mother has had. It readily fixes and activates complement-mediated cell lysis and is the major isotype involved in ADCC and opsonization (i.e., it coats cells that express the antigens they are directed against and attracts phagocytes to said cells).|
|IgM||A primarily pentameric isotype (although a monomeric form does exist, but it mostly serves as a B-cell antigen receptor) that is the first isotype produced by B-cells, before they switch to other isotypes. It plays a crucial role in agglutination as its larger struction makes it comparatively easy for it to agglutinate cells. It also readily fixes and activates complement.|
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- ↑ For example, if it is a toxin, as a change in the structure of the toxin caused by the binding of the antigen makes it unable to act at its biochemical target (e.g., receptors or enzymes), etc.
- ↑ Where the antigens on multiple cells are bound by the one antigen leading to the cells clumping together
- ↑ When the antigen are free, that is, for example, toxins, and clump together due to the same mechanisms seen in agglutination
- ↑ That is, the complement-binding sites on their stem regions align and attract complement proteins which in turn lyse the cells on which these antibodies have bound to).
- ↑ Janeway, CR, Jr; Travers, P; Walport, M; Shlomchik, MJ (2001). "The destruction of antibody-coated pathogens via Fc receptors". Immunobiology: The Immune System in Health and Disease. 5th edition. New York, USA: Garland Science. ISBN 978-0815336426. Unknown parameter
- ↑ Marieb, EN; Hoehn, K (2013). "Chapter 21 The Immune System: Innate and Adaptive Body Defenses". Human Anatomy & Physiology (9th ed.). Boston, USA: Pearson. pp. 781–782. ISBN 978-0-321-74326-8.
- ↑ Kumar, V; Abbas, AK; Aster, AC (July 2014). "Chapter 6 Diseases of the Immune System". Robbins & Cotran Pathologic Basis of Disease 9e. Philadelphia, USA: Saunders. p. 192. ISBN 978-0-8089-2450-0.
- ↑ Marieb, EN; Hoehn, K (2013). "Chapter 21 The Immune System: Innate and Adaptive Body Defenses". Human Anatomy & Physiology (9th ed.). Boston, USA: Pearson. p. 782. ISBN 978-0-321-74326-8.
- ↑ 5.0 5.1 5.2 Janeway, CR, Jr; Travers, P; Walport, M; Shlomchik, MJ (2001). "The distribution and functions of immunoglobulin isotypes". Immunobiology: The Immune System in Health and Disease. 5th edition. New York, USA: Garland Science. ISBN 978-0815336426. Unknown parameter
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 Marieb, EN; Hoehn, K (2013). "Chapter 21 The Immune System: Innate and Adaptive Body Defenses". Human Anatomy & Physiology (9th ed.). Boston, USA: Pearson. p. 783. ISBN 978-0-321-74326-8.