Passive (Indirect) Agglutination
Definition
An agglutination format in which a soluble antigen or antibody is adsorbed or covalently linked to a carrier particle (latex bead, tanned red cell, or colloidal gold). Crosslinking of coated particles by the corresponding antibody or antigen produces the same visible clump as direct agglutination, extending the principle to any molecule that can be attached to a surface.
- Carrier
- Latex, tanned red cells, or colloidal gold
- Mechanism
- Antigen or antibody adsorbed to a particle
- Contrast
- Direct agglutination uses native cell-surface antigen
- Benefit
- Extends testing to any attachable molecule
Common questions
How does passive agglutination differ from direct agglutination?+
Direct agglutination uses the antigen naturally present on a cell's surface, such as red blood cells in ABO typing, while passive agglutination coats an inert carrier particle so molecules with no natural particulate form can still be tested this way.
Why might colloidal gold be chosen as a carrier over latex?+
Colloidal gold produces a strong visible colour change at low concentrations, which is useful in rapid lateral-flow style tests where sensitivity and a clear visual readout matter.
Related terms
- Agglutination Titre
- The reciprocal of the highest dilution of a serum or antigen solution at which visible agglutination still occurs. A titre of 1:64...
- Antiglobulin (Coombs) Test
- A two-stage assay that detects IgG antibodies bound to red blood cells. The direct test detects in-vivo cell-bound IgG; the indirect test...
- Hemagglutination
- Agglutination specifically involving red blood cells as the antigen-bearing particles. Direct hemagglutination occurs when antibodies bind surface antigens naturally present on red...
- Low-Ionic-Strength Saline (LISS)
- A reaction medium with reduced salt concentration used to enhance agglutination sensitivity, particularly for IgG antibodies. Lowering ionic strength reduces the electrostatic...
- Prozone (Hook) Effect
- A false-negative agglutination result caused by antibody excess. When antibody molecules outnumber antigen sites, each site is occupied by a single antibody...