Agglutination

• Sensitive than precipitation reaction for detecting antibodies.
• Agglutination is a two-step reaction.

The primary reaction:

• Binding of the Fab portion of antibody to antigenic determinants on the particulate antigen.

The secondary reaction:

• Cross-linking by multivalent antibodies, of the particles of multivalent antigen to form aggregates.
• The most obvious clinical application of agglutination is blood-typing;
• erythrocyte blood group surface antigens (e.g., the ABO blood group factors) 
• react with antibody that is specific for these allo-antigens to produce agglutination.
• The cross- matching of blood depends upon this technique.

Agglutination is of following types:

Slide agglutination

• used for blood grouping and cross matching.

Tube agglutination

• Widal test Brucellosis
• Weil-Felix reaction
• Paul Bunnel test
• cold agglutination
• Streptococczs MG test.

Antiglobulin

• Coombs test
• used for detecting incomplete Ab of brucellosis
• anti- Rh Ab.

 Passive agglutination test

• used to detect Ab
• by adsorbing soluble Ag on carrier particles so precipitation reaction converts into agglutination test 
• more convenient and more sensitive.
• e.g. Rose waller test, test detecting RA factor by using amboceptor.

Latex agglutination test

• latexfixation test
• for detection of ASO, CRP, RA factor
• HCG; Streptozyme test.
• Rapid test for acute Pyogenic meningitis

Reversed passive agglutination 

• Estimation of antigen by adsorbing antibody to carrier particles.

Heterophilic agglutination reaction

• Some organisms of different class or species share closely related antigens.
• When serum containing agglutinin (antibody) of one organism gives agglutination reaction with antigen of other organism.
• Examples are
• Streptococcus M.G. agglutination test for primary atypical pneumonia.
• Weil – Felix reaction for typhus fever
• Paul Bunnell test fin- IMN.