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Jai Ganesh

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Agglutination

Agglutination

Gist

Agglutination is the clumping together of particles, typically cells like red blood cells or bacteria, in a liquid due to a reaction between antigens and antibodies. This process is a key aspect of the immune system, helping to identify and neutralize foreign invaders.

Agglutination, which refers to the clumping of particles together, is an antigen-antibody reaction that occurs when an antigen, a molecule capable of triggering the adaptive immune response, is mixed with its corresponding antibody at a suitable pH and temperature.

Summary:

Agglutination: Reactions, Types, Tests, Applications

Agglutination is an antigen-antibody reaction in which a particulate antigen combines with its antibody in the presence of electrolytes at a specified temperature and pH resulting in the formation of visible clumping of particles.

It occurs optimally when antigens and antibodies react in equivalent proportions. This reaction is analogous to the precipitation reaction in that antibodies act as a bridge to form a lattice network of antibodies and the cells that carry the antigen on their surface. Because cells are so much larger than a soluble antigen, the result is more visible when the cells aggregate into clumps.

When particulate antigens react with specific antibody, antigen-antibody complex forms visible clumping under optimum PH and temperature. Such a reaction is called agglutination. Antibodies that produce such reactions are called agglutinins.

What is Agglutination?

Agglutination is the visible expression of the aggregation of antigens and antibodies. Agglutination reactions apply to particulate test antigens that have been conjugated to a carrier. The carrier could be artificial (such as latex or charcoal particles) or biological (such as red blood cells). These conjugated particles are reacted with patient serum presumably containing antibodies. The endpoint of the test is the observation of clumps resulting from that antigen-antibody complex formation. The quality of the result is determined by the time of incubation with the antibody source, amount and avidity of the antigen conjugated to the carrier, and conditions of the test environment (e.g., pH and protein concentration). Various methods of agglutination are used in diagnostic immunology and these include latex agglutination, flocculation tests, direct bacterial agglutination, and hemagglutination.

Agglutination differs from precipitation reaction in that since agglutination reaction takes place at the surface of the particle involved, the antigen must be exposed and be able to bind with the antibody to produce visible clumps. In agglutination reactions, serial dilutions of the antibody solution are made and a constant amount of particulate antigen is added to serially diluted antibody solutions. After several hours of incubation at 37°C, clumping is recorded by visual inspection. The titer of the antiserum is recorded as the reciprocal of the highest dilution that causes clumping. Since the cells have many antigenic determinants on their surface, the phenomenon of antibody excess is rarely encountered.

Prozone Phenomenon

The condition of excess antibody, however, is called a prozone phenomenon. At a high concentration of antibody, the number of epitopes are outnumbered by antigen-binding sites. This results in the univalent binding of antigen by antibody rather than multivalently and thus, interferes in the crosslinking of antigen (Lattice formation).

Occasionally, antibodies are formed that react with the antigenic determinants of a cell but does not cause any agglutination. They inhibit the agglutination by the complete antibodies added subsequently. Such antibodies are called blocking antibodies. Anti-Rh antibodies and anti-brucella antibodies are few examples of such blocking antibodies.

Agglutination tests are easy to perform and in some cases are the most sensitive tests currently available. These tests have a wide range of applications in the clinical diagnosis of non- infectious immune disorders and infectious diseases. Agglutination reactions have a wide variety of applications in the detection of both antigens and antibodies in serum and other body fluids. They are very sensitive and the result of the test can be read visually with ease.

Details

Agglutination is the clumping of particles. The word agglutination comes from the Latin agglutinare (glueing to).

Agglutination is a reaction in which particles (as red blood cells or bacteria) suspended in a liquid collect into clumps usually as a response to a specific antibody.

This occurs in biology in two main examples:

* The clumping of cells such as bacteria or red blood cells in the presence of an antibody or complement. The antibody or other molecule binds multiple particles and joins them, creating a large complex. This increases the efficacy of microbial elimination by phagocytosis as large clumps of bacteria can be eliminated in one pass, versus the elimination of single microbial antigens.
* When people are given blood transfusions of the wrong blood group, the antibodies react with the incorrectly transfused blood group and as a result, the erythrocytes clump up and stick together causing them to agglutinate. The coalescing of small particles that are suspended in a solution; these larger masses are then (usually) precipitated.

In immunohematology:

Hemagglutination

Hemagglutination is the process by which red blood cells agglutinate, meaning clump or clog. The agglutin involved in hemagglutination is called hemagglutinin. In cross-matching, donor red blood cells and the recipient's serum or plasma are incubated together. If agglutination occurs, this indicates that the donor and recipient blood types are incompatible.

When a person produces antibodies against their own red blood cells, as in cold agglutinin disease and other autoimmune conditions, the cells may agglutinate spontaneously. This is called autoagglutination and it can interfere with laboratory tests such as blood typing and the complete blood count.

Leukoagglutination

Leukoagglutination occurs when the particles involved are white blood cells.

An example is the PH-L form of phytohaemagglutinin.

In microbiology

Agglutination is commonly used as a method of identifying specific bacterial antigens and the identity of such bacteria, and therefore is an important technique in diagnosis.

History of discoveries

Two bacteriologists, Herbert Edward Durham (1866-1945) and Max von Gruber (1853–1927), discovered specific agglutination in 1896. The clumping became known as Gruber-Durham reaction. Gruber introduced the term agglutinin (from the Latin) for any substance that caused agglutination of cells.

French physician Fernand Widal (1862–1929) put Gruber and Durham's discovery to practical use later in 1896, using the reaction as the basis for a test for typhoid fever. Widal found that blood serum from a typhoid carrier caused a culture of typhoid bacteria to clump, whereas serum from a typhoid-free person did not. This Widal test was the first example of serum diagnosis.

Austrian physician Karl Landsteiner found another important practical application of the agglutination reaction in 1900. Landsteiner's agglutination tests and his discovery of ABO blood groups was the start of the science of blood transfusion and serology which has made transfusion possible and safer.

Additional Information

The process in which free red blood cells are bound together by an antibody and reduced to a visible pellet when centrifuged, most typically in test tubes. In short, when an antibody binds to an RBC antigen then binds to an antigen on a second RBC, the antibody links form “bridges” that lead to a visible aggregate of RBCs. Agglutination is the central reaction in blood banking, as most of our testing for decades has relied on its detection. The agglutinates are typically characterized on a 0-4+ scale, with 0 representing no reaction, and 4+ indicating a very strong reaction. Antibodies vary in their ability to cause agglutination, as IgM antibodies agglutinate RBCs carrying target antigens quite efficiently, while IgG antibodies typically bind to incompatible RBCs but do not directly agglutinate them. The characteristics of the agglutination reaction are used, usually reliably, to predict whether or not a particular antibody will cause a problem for a patient in-vivo. Agglutination is synonymous to the more precise but less commonly used “hemagglutination.”

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