Agglutination

Agglutination is the visible clumping of cells or particles when antibodies bind antigens on their surfaces and cross-link them.

Antibody: An antibody is an immune-system protein that binds specific molecular structures called antigens. Its multiple binding sites can connect antigen-bearing particles into a clump.
ABO blood group system: The ABO blood group system classifies red blood cells by the A and B antigens on their surfaces. Antibodies against incompatible ABO antigens cause rapid red-cell agglutination.
Precipitation reaction: A precipitation reaction forms an insoluble network when soluble antigens bind multivalent antibodies. It resembles agglutination but produces an insoluble precipitate from soluble reactants.
Immunoglobulin G: Immunoglobulin G is the most abundant antibody class in human blood and extracellular fluid. Its two antigen-binding arms can cross-link targets, though some IgG reactions need enhancement.
Antigen: An antigen is a molecule or molecular region that can be specifically recognized by an antibody or immune receptor. Antigens on particle surfaces provide the binding sites that agglutination links.
Hemagglutination assay: A hemagglutination assay detects substances by observing whether red blood cells form a visible pattern or clump. It uses red-cell agglutination as a direct readout of antibody or viral activity.
Flocculation test: A flocculation test detects antigen–antibody reactions through fine, suspended aggregates called floccules. Its fine suspended aggregates differ in appearance from the larger clumps typical of agglutination.
Immunoglobulin M: Immunoglobulin M is a large antibody class commonly secreted as a pentamer with ten antigen-binding sites. Its high valency makes it particularly effective at directly agglutinating cells.
Antibody valency: Antibody valency is the number of antigen-binding sites available on an antibody molecule. Multiple binding sites let antibodies bridge separate targets rather than bind only one.
Latex agglutination test: A latex agglutination test detects an analyte using latex beads coated with antibodies or antigens. The beads amplify molecular recognition into clumps visible to the eye.

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