Agglutination Reactions

Agglutination is defined as the antigen-antibody reaction in which antibodies cross-link particulate antigens resulting in the visible clumping of particles. Antibodies that show such reactions are called agglutinins.

Agglutination reactions work on the principle of cross-linking of the polyvalent antigens. Following are the advantages of agglutination reactions: 

1. easy to perform 
2. require no expensive equipment, and 
3. detects antibody concentrations as low as nanograms per milliliter. 

Types of agglutination reactions: 

1. Hemagglutination:
   type of agglutination reaction is routinely performed to type red blood cells (RBCs), wherein RBCs are mixed with antisera to the A or B blood-group antigens on a slide. The presence of antigen on the cell surface is proved by forming a visible clump on the slide. This RBC typing forms the basis for matching blood types for transfusions.
   
2. Bacterial Agglutination:

This type of agglutination reaction is performed to diagnose infection and provide a way to type bacteria. Any bacterial infection elicits the production of serum antibodies within the host. 

These serum antibodies are specific for surface antigens on the bacterial cells, which bacterial agglutination reactions can detect.

Bacteria is added to the previously serial diluted array of tubes containing serum from a patient thought to be infected with a given bacterium. The last tube, which shows visible agglutination, reflects the serum antibody titre of the patient. Thus, the agglutinin titre is the reciprocal of the greatest serum dilution that elicits a positive agglutination reaction.

3. Active agglutination 

In this type of agglutination, epitopes of interest are naturally found on a test particle, such as antigens found on RBCs, bacterial and fungal cells.

Examples –

1. Blood grouping and cross-matching
2. Widal test for diagnosis of typhoid fever 
3. Brucella agglutination test for Brucellosis 
4. Weil Felix test for Rickettsiosis
5. Passive agglutination: 

Passive agglutination is useful when the epitope of interest does not occur naturally on the cells or particles to be agglutinated. The epitopes or soluble antigens are chemically fixed to carrier particles such as – latex, polystyrene, bentonite.

 Passive agglutination is also useful when pathogen culture is not feasible, e.g., viral diseases. 

Synthetic beads offer better consistency, uniformity, and stability. In addition, those agglutination reactions which employ synthetic beads are rapidly read within 3 to 5 minutes of mixing the beads with the test sample.  

Article by- SAMPRATI PAREKH (MSIWM049).

References:

1. Cellular and Molecular Immunology by Abul K. Abbas – 7^thEdition 
2. Kuby Immunology – 5^thEdition