When a particulate antigen is mixed with its antibody in the pres-ence of electrolytes at a suitable temperature and pH, the particles are clumped together or agglutinated.
Agglutination is more sensitive than precipitation for the detec-tion of antibodies. The same principles govern agglutination and pre-cipitation. Agglutination occurs optimally when antigens and antibodies react in equivalent proportions. The zone phenomoenon may be seen when either an antibody or antigen is in excess.
Incomplete or monovalent antibodies do not cause agglutina-tion, though they combine with the antigen. They may act as ‘blocking antibodies’, inhibiting agglutination by the complete antibody added subsequently.
1. Slide Agglutination
2. Tube Agglutination
3. Passive Agglutination
When a drop of the appropriate antiserum is added to a smooth, uniform suspension of a particulate antigen in the drop of a saline on a slide or tile, agglutination takes place. A positive result is indicated by the clumping together of the particles and the clearing of the drop. Mix-ing the antigen and the antiserum with a loop or gently rocking the slide facilitates the reaction. Depending on the titer of the serum, agglutina-tion may occur instantly or within seconds. Clumping occurring after a minute may be due to drying of the fluid and should be disregarded.
It is essential to have on the same slide a control consisting of the antigen suspension in saline, without the antiserum, to ensure that the antigen is not agglutinable. Agglutination is usually visible to the naked eye but may sometimes require confirmation under the microscope. Slide agglutination is a routine procedure for the identification of many bacterial isolates from clinical specimens. It is also the method used for blood grouping and cross matching.
This is a standard quantitative method for the measurement of antibodies. When a fixed volume of a particulate antigen suspension is added to an equal volume of serial dilution of an antiserum in test tubes, the agglutination titer of the serum can be estimated. Tube agglutination is routinely employed for the serological diagnosis of typhoid, brucello-sis and rickettsial fever. Widal agglutination test is done for typhoid, Brucella agglutination test is done for Brucellosis and Weil Felix test is done for rickettsial infections
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