What are the Types of Antigen-Antibody (AG-AB) Reactions?

1. Agglutination Reactions:

Aggregation of cells due to antibody binding is known as "Agglutination". The word agglutination is derived from Latin word agglutinate, means "to glue to."

In humus, binding of Abs pulls the antigen bearing cells close to each other resulting in the formation of clumps. The antibodies that cause agglutination of cells are called "agglutinins" and the antigens aggregated are called "agglutinates".

Mechanism of Agglutination:

The bivalent or multivalent antibodies can bind with two or more antigens at a time. When Ab binds to more than one antigen present on different cells, the individual antigen molecules brought close to each other, resulting in the formation of clumps.

Since IgM antibody has more number of antigen binding sites it is a more effective agglutinin. Agglutination of blood group antigens by IgM antibodies in mismatched blood transfusion is responsible for the blood transfusion reactions.

Agglutination test has wide application in clinical field. For example agglutination test is used to test blood groups, and infectious diseases such as typhoid, leptospirosis, malaria, trypanosomiasis, pneumonia etc.

2. Precipitation and Flocculation Reaction:

When an antibody binds to a soluble antigen, the antigen becomes insoluble and it may precipitate or float in the fluids. If Ag - Ab complex precipitates, it is referred as "precipitation reaction".

Some times the Ag-Ab complex may float instead of precipitation; in that case the reaction is called as "flocculation reaction" and the Ag-Ab complex is known as "floccule".

Mechanism of Precipitation and Flocculation Reaction:

Depending upon its valency a single antibody can bind to more than two antigens at a time. For example IgM Ab with valency 10 can bind to a maximum of 10 Ags at a time, if it happens to get suitable epitopes in its surroundings.

Since IgG antibodies valency is two they can bind to two antigens only. When a single antibody binds to more than one antigen a bridge between the soluble antigens develops and the grouped soluble antigens fail to dissolve in body fluids resulting their precipitation or flocculation.

However, the amount of precipitate and consequently it's' visibility directly depends on the quantities of both antigen and antibody and their ratio. When the Ag, Ab proportion is optimum the precipitate formation shows the maximum level. If the amount of antibody is excess than Ags, precipitation formation is not at maximum level and the serum is not saturated.

When antigen is in excess only, small antigen-antibody complexes are able to form and the small complexes are fairly soluble. Since the Ags are excess, the available Abs may not be sufficient to form a lattice, necessary for precipitation or flocculation.

3. Complement Fixation:

Antibody binding with antigen is not sufficient to remove the antigen from body. Hence Ag-Ab complex initiates activation and binding of complement system to it. Binding of complement system to Ag-Ab complex is known as complement fixation.

The activated complement enzymes continue further immune reaction that finally removes the foreign agent (antigen).

Mechanism of Complement Fixation:

Inactive enzymes of complement system get activated in a cascade manner in the presence of Ab- Ag complex, and bind to the pathogen to form membrane associated complex- MAC.

The complement fixation assay can be used to check presence of specific antibody or specific antigen in an individual's serum. It was widely used to diagnose infections, particularly microbial infections and rheumatic diseases that are not easily detected by culture methods.

But now new serological methods such as ELISA, PCR and DNA-based methods of pathogen detection are in use for clinical diagnosis.

4. Opsonization:

Innate immunity of an individual keeps constant vigil on foreign agents entering in to the body and the accessory cells of immune system phagocytise the foreign agents to save the body from them.

However certain antigens escape the phagocytic action through various ways. If antibody binds to such antigen, it becomes susceptible for phagocytosis. This process is known as "Opsonization".

The antibody that is making the antigen susceptible for pagocytosis is known as "opsonin".

Mechanism of Opsonization:

The antibodies or opsonins bind to the surface antigens of bacteria. The antigen antibody complex in turn initiates complement system.

Phagocytes with receptors for Fc region of antibody moves towards Ag Ab complex through chemotactic action of activated complement system factors C2a, C4a etc. and attach to the bacteria through Fc region of antibody. Either phagocytosis or MAC formation on the surface of bacteria results in lyses and removal of bacteria.