Serology is the in vitro study and clinical utilization of antigen-antibody interactions.

The agglutination reaction after antigen have bound to their specific antibodies, is widely used clinically; for example, for erythrocyte typing in blood banks, diagnosing auto-immune haemolytic disorders and detecting rheumatoid factor.

Immunoassays are used to quantify small amounts of antigen, antibody or antigen-antibody complexes. Radio-immunoassays (RIA), in which antigen is labelled with a

Figure 11.10. Schematic representation of an immunoglobulin molecule. Antigen is bound by the Fab regions whereas the Fc region mediates the biological activity such as opsonization.

Antibody recognition Adhesion Engulfing

Figure 11.11. A diagrammatic representation of phagocytosis. The antigens on a particle is coated with antibodies. The phagocytic cell (macrophage or neutrophil) have receptors for the Fc portions of the immunoglobulin molecules on the particle. This aid with phagocytosis is called opsonization (from Benjamini et at., 1996).

radioactive isotope, is sensitive for extremely small amounts of antigen and antibody. Enzyme-linked immunosorbent assay (ELISA), a solid-phase immunoassay in which antigen is coated to a plastic surface and antibodies are labelled with a colour reagent, can be used for qualitative or quantitative determination of antigen or antibody.

Immunofluorescence is a method of detecting antigen by labelling antibody with fluorescent compounds. Fluorescein isothiocyanate (FITC), which fluoresces with a visible greenish colour, is one of the most commonly used fluorescent compounds in immunology.

Monoclonal antibodies are homogenous populations of antibody molecules, derived from a single lymphocyte, in which all antibodies are identical and of the same precise specificity for a given epitope. These monoclonal antibodies are used both for diagnostic purposes and as 'magic bullets' in immunotherapy of cancer. Most monoclonal antibodies are made in mouse cells, but with gene technology, in vitro production has become easier.

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