The Immune Response: Antigen-Antibody Reactions. Introduction. Antibodies are bifunctional - they bind to the target antigen they recognize as foreign, and they enable other defense components to react with it Variable domain (Fab) - binds to target antigen
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Antibody-dependent cell-mediated cytotoxicity - involves the complement system, NK cells, or release of cytotoxic mediators from effector cells that attach to the target cell by means of Fc receptors
IgE and parasitic infection - in the presence of elevated IgE levels, eosinophils bind parasites and release lysosomal enzymes
Prepares the microorganism for phagocytosis
Phagocytes recognize the Fc portion of IgG or IgM antibodies coating the surface of the foreign microorganism
Phagocytosis can also be stimulated by components of the complement system, whether initiated by the classical or alternative pathways
Inflammation can be mediated by IgE attachment to mast cells and basophils, or by the binding of one of the complement components to mast cells and platelets
This complement component is also a powerful chemoattractant for macrophages, neutrophils, and basophils
Immune complex formation - two or more antigen-binding sites per antibody molecule lead to cross-linking, forming precipitins (molecular aggregates) or agglutinins (cellular aggregates)
Agglutination that specifically involves red blood cells is called hemagglutination
In vivo testing involves immediate or delayed skin testing for the presence of antibodies to various antigens
Antigen-Antibody Reactions In Vitro (serology)
Agglutination - visible clumps or aggregates of cells or of coated latex microspheres
If red blood cells are agglutinated, this is called hemagglutination
Agglutination inhibition can be used to detect serum antibodies or to detect the presence of specific substances (e.g., illegal drugs) in urine samples by a competition assay
Rapid urine testing for drugs, e.g cocaine – Abuscreen method
Figure 33.9 – Latex agglutination test for pregnancy
Positive pregnancy test
Negative pregnancy test
Figure 33.10 – Viral Hemagglutination
Some viruses bind to RBC and cause hemagglutination. If serum contains anti-viral Abs, then hemagglutination is inhibition – positive test.
Figure 33.11: Tube agglutination test for determining antibody titer.
Q: What is the titer in rows A-H?
Figure 33.11: A microtiter plate illustrating hemagglutination. Ab in wells 1-10. Positive control = row 11, negative control = row 12. RBCs added to each well. If enough Ab is available to agglutinate the cells, they bind as a mat to the bottom of the well. If insufficient Ab is available, they form a pellet at the bottom.
Irreversible alterations to complement components that are initiated by the binding of antibody to antigen;
Used to detect the presence of serum antibodies, thereby indicating prior exposure to a pathogen
If immune complexes are formed, then complement is used up and lysis will not occur when sensitive indicator cells are added
If immune complexes are not formed, then complement is not used up and lysis will occur when sensitive indicator cells are added