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Agglutination Definition

Agglutination refers to the clumping or adhesion of small particles that are suspended in a solution. This biological process can be observed in the presence of agglutinins that induce this phenomenon. For example, lectins and antibodies are a type of agglutinins.

What is Agglutination?

In the case of immunology and microbiology, Agglutination process refers to the clumping of the bacterial cells when a complement antibody is present in the media. It can refer to the process where foreign substances clump as an immunological reaction to impede them from interfering with physiological processes making agglutination a crucial response.

Agglutination process has several clinical implications as it is immunologically a significant process. One of the basic medical importance lies in blood transfusions. If a mismatched blood transfusion can lead to agglutination that may be highly dangerous to the recipient.

Agglutination Examples

i. Agglutination of Red Blood Cells

Red blood cells agglutination is referred to as hemagglutination. The agglutinins in this case are known as hemagglutinins that cause clumping of RBCs. They are comprised of glycoproteins present on the surface of RBCs. Certain dietary lectins like Phytohemagglutinin (PHA) present in legumes causes RBCs to stick together and cause agglutination. Ingesting uncooked red kidney beans may lead to hemagglutination as they are rich in PHA.

Agglutination, Agglutination Definition, What is Agglutination, 1
ii. Agglutination of Bacteria

Microbes also show the phenomenon of agglutination when they bind with some specific antibodies. Agglutinins react with antibodies resulting in clumping or agglutination.

iii. Agglutination: Bacterial Opsonization

Agglutination can occur in the human body when foreign bodies like bacteria gain entry and are eventually detected by the immune system especially the antibodies like IgM, which causes agglutination in bacteria. The antibodies bind with their epitopes to bacteria. The skin provides a barrier and restricts the entry of microbes but despite this microbes can gain access to the body.

Opsonization is one of the defense mechanisms of the immune system. In this, a coat of antibodies is formed over the microbes making them sticky or adhesive resulting in the formation of clumps. The microbial antibody compound is later cleared by the phagocytic action of macrophages.

Laboratory Applications of Agglutination

The concept of hemagglutination can be applied for the detection of blood type in a person. There are 4 possible blood groups in the case of humans. These are the O, AB, A, and B blood group. The blood group is decided by the glycoprotein antigen present on the RBC surface. The typing method determines the antigen present by utilizing commercially produced antibodies against them.

This test also helps to analyze the presence of the Rh factor which is another cell surface antigen present on RBCs. The blood types are further grouped based on the presence or absence of RBC as Rh-negative or Rh-positive. In the blood typing card, the blood of the patient is mixed with different prepared antibodies that detect the presence of a specific antigen.

Agglutination reaction on coming in contact with an antibody specifies that the particular antigen is present. For instance, if blood agglutinates with anti-B antibodies but not with anti-B and anti-Rh antibodies then the blood group of the person is A- blood type.

Agglutination: Blood Typing

If the blood card is analyzed under a microscope, we can see the clumping in cells where agglutination has occurred. This is a direct method of determining the blood type of a person and is termed forward blood typing. There is another method that is indirect and is known as is back-typing. For this method, the first serum is collected and is analyzed for the presence of antibodies by reacting with different blood types.

For example, if the person has blood type B then they have anti-A antibodies. When analyzed it will react with A blood group blood to cause agglutination. The serum from the patient will be tested for the presence of antibodies that will react to known blood types. Following the above example, a blood type A individual will have a serum containing anti-B antibodies. Thus, the serum sample will cause type B blood to stick together but not type A blood.

It is crucial to analyze the blood type before transfusion. Besides transfusion, other clinical applications of hemagglutination include hemagglutination inhibition assay and hemagglutination assay. These work on the same principle and are used to measure bacteria, antibodies, and viruses in the serum.

Agglutination vs Coagulation

Agglutination and coagulation refer to the clumping of particles in a suspension but they differ in terms of complexity. Agglutination results in clumping of RBCs due to immunological reaction. While coagulation is a more complicated process resulting in the formation of a platelet plug that later forms a fibrin clot.

Agglutination in general forms a part of coagulation as the adhesion of cells increases in clot formation. An injury to the wall of blood vessels activates platelets that adhere to one another due to the von Willebrand factor leading to the formation of a plug after which a fibrin mesh is deposited that leads to clot formation.


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