EMS World

AUG 2011

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CE ARTICLE Table 1: Classes of Antibodies ANTIBODY CLASS FUNCTION IgA IgD IgE IgG COMMENTS Eliminates pathogens prior to entry into the body. Binds antigens located in bodily fl uids to B cells that have not been previously exposed to antigens. Binds antigens and triggers histamine release from mast cells and basophiles. Provides the majority of antibody-based immunity against antigens. Found in mucosal areas such as the gut, respiratory tract and urogenital tract. Also found in glandular secretions such as tears, saliva and mucus. Function poorly understood; may play a role in activation of B cells. Has major role in allergic response. The only antigen capable of crossing the placental barrier, responsible for passive immunity in infants. Largest class of antibodies; makes up about 80% of all antibodies in humans. IgM Binds with pathogens immune to IgG attack. Primary site of synthesis is the spleen. Anti-A and anti-B forms of IgM responsible for reactions between incompatible blood types. cell-mediated immunity, or cellular immu- nity. B cells provide a defense against pathogens located in body fl uids in a process termed antibody-mediated immu- nity, or humoral immunity. In cell-mediated immunity, diseased cells are identifi ed by cytotoxic T cells, or killer T cells, which then divide to produce more cytotoxic T cells and memory cells. Cytotoxic T cells destroy their targets by one of three ways: 1) by releasing perforin, which perforates the target cell’s cellular membrane; 2) by secreting lymphotoxin, which disrupts the target cell’s metabo- lism; or 3) by inducing apoptosis (geneti- cally programmed cellular death) in cells infected with virus or bacteria, and in tumor cells. Memory T cells will remain in circulation to be used should the pathogen reappear in the future. Another type of T cell is the helper T cell, which stimulates both cell-mediated and anti- body-mediated immunity when activated. In comparison, antibody-mediated immunity is involved in a complex series of events initiated by the formation of antigen-antibody complexes that results in a sophisticated chemical attack on specifi c antigens. It relies on the relation- ship between antigens and antibodies. An antigen is any substance that results in the formation of an antibody by the immune system. Typical antigens include proteins found within bacteria, viruses and fungi. An antibody, also known as an immunoglobin, is a “Y”-shaped protein 46 AUGUST 2011 | EMSWORLD.com produced by B cells and used to identify and neutralize specifi c antigens. There are fi ve classes of antibodies: IgA, IgD, IgE, IgG and IgM (see Table 1). The human body has millions of different types of B cells, each with a unique antibody on its cell membrane. When a matching antigen appears in the body fl uids, it binds to its respective B cell; at this time, the B cell is said to be sensitized. The B cell is then stimu- lated by helper T cells to differentiate into memory B cells and activated B cells, which produce and secrete antibodies specifi c to the antigen. The free-fl oating antibodies encounter and bind to their antigen, forming an antibody-antigen complex, which results in the elimination of the antigen in one or more of several ways. Antibodies can bind many antigens together, a process termed agglutination, which allows for easier recognition and disposal. Antibody-antigen complexes attract and enhance the effi cacy of cells such as macrophages, eosinophils and neutrophils, which phagocytize (ingest) the antigen. In addition, the antibody- antigen complexes promote infl amma- tion via the stimulation of mast cells and basophiles, which release chemicals such as histamine. HYPERSENSITIVITY Hypersensitivity, or a hypersensitivity reaction, is an immune response that is excessive and produces an undesirable reaction that can be uncomfortable, harmful and occasionally fatal. For a hypersensitivity reaction to occur, a host must have been previously exposed to an antigen and sensitized. A four-group clas- sifi cation system for hypersensitivity reac- tions was created and published in 1963 and is still in use today (see Table 2).1 Allergic Reactions and Anaphylaxis The term allergy refers to an IgE-mediated (Type 1) hypersensitivity reaction; the antigens that result in an allergic reaction (atopy) are termed aller- gens. Common allergens include foods (eggs, peanuts, shellfi sh, many others), antibiotics, local anesthetics and other medications, insect stings (hymenop- tera venom, fi re ant bites), latex and hormones (insulin, methylprednisolone, progesterone). The mildest form of a Type 1 hyper- sensitivity reaction is an allergic, or atopic, reaction. Patients are considered atopic when they have a predisposition towards hypersensitive responses. Atopy is a local- ized reaction that occurs after the host is exposed to an allergen to which it is sensitized. During the second exposure, histamine released from mast cells and basophiles results in localized increased capillary permeability, smooth muscle contraction and sensory nerve stimulation. When released locally these effects give rise to signs and symp- vasodilation,

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