Both the membrane of a phagocytosing cell, as well as its target, have a negative charge (zeta-potential), making it difficult for the two cells to come close together. During the process of opsonization, antigens are bound by antibody and/or complement molecules. Phagocytic cells express receptors that bind opsonin molecules. These include the Fc receptors. With the antigen coated in these molecules, binding of the antigen to the phagocyte is greatly enhanced. Most phagocytic binding cannot occur without opsonization of the antigen.
Furthermore, opsonization of the antigen and subsequent binding to an activated phagocyte will cause increased expression of complement receptors on neighboring phagocytes.
Examples of opsonin molecules include:
- antibodies: IgG and IgA
- components of the complement system: C3b, C4b, and iC3b
- Mannose-binding lectin (initiates the formation of C3b)
The most important are IgG and C3b.
Immune system / Immunology
|Systems||Adaptive immune system vs. Innate immune system • Humoral immune system vs. Cellular immune system • Complement system (Anaphylatoxins) • Intrinsic immune system|
|Antibodies and antigens||Antibody (Monoclonal antibodies, Polyclonal antibodies, Autoantibody) • Allotype • Isotype • Idiotype • Antigen (Superantigen)|
|Immune cells||White blood cells (T cell, B cell, NK cell, Mast cell, Basophil, Eosinophil) • Phagocyte (Neutrophil, Macrophage, Dendritic cell) • Antigen-presenting cell • Reticuloendothelial system|
|Immunity vs. tolerance||Immunity • Autoimmunity • Allergy • Tolerance (Central) • Immunodeficiency|
|Immunogenetics||Somatic hypermutation • V(D)J recombination • Immunoglobulin class switching • MHC / HLA|
|Substances||Cytokines • Opsonin • Cytolysin|
|Other||Inflammation • Epitope (Hapten) • Cross-reactivity|
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