Antibodies/immunoglobulins (Igs)/gamma globulins
Group of structurally similar serum immunoglobulins secreted by activated B cells or plasma cells that mediate humoral antibody. Characteristic feature is their ability to recognise specific antigens.
Five types/classes – IgG, IgM, IgA, IgE and IgD
Common features:
All have the same light chains (either kappa or lambda) – the heavy chains are specific to each type.
Each made up of a chain of domains of 110 amino acids.
Each can be cleaved enzymatically into two fragments – Fc and Fab fragments
IgM:
• composed of five basic units held together by J-chain
• functions to neutralise microorganisms
• complement activator promotes target cell lysis
• first to appear in response to antigen or after immunisation
IgG:
• smallest molecular weight of immunoglobulins
• produced in small amounts initially, but on subsequent exposure boost production to large amounts
• fixes complement promotes target cell lysis
• binds to target cells to enhance phagocytosis – Fc receptors for IgG exist on macrophages, PMN’s, lymphocytes, eosinophils and platelets
• Can cross placenta into foetus neonatal immunity
• coats bacteria facilitate phagocytosis
IgA:
• mainly in mucosal secretions (GI tract and lungs) and milk (can be transferred to foetus)
IgE:
• present in trace amounts in serum
• locally attached to mast cells release of histamine, heparin and other mediators
• mediates allergic reactions
IgD:
• cell membrane bound immunoglobulin on surface of mature B-cells
• participates in activation of B-cells by acting as a receptor for antigen recognition
• not released into serum
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