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In biology, agglutination refers to the bunching together of particles. This process is especially important as part of the immune system, the response process that an organism uses to fight disease. Hemagglutination, the clumping of red blood cells, has special applications in medicine, where it is used to determine blood type and find the concentration of infecting viruses or bacteria in the bloodstream.
Foreign bacteria or viruses entering the body contain specific components called antigens, which trigger an immune response in the host. White blood cells in the body produce proteins known as antibodies in response to the presence of antigens. The antibodies bind with the antigens through a structural mechanism similar to a key and a lock, and can either neutralize the antigen directly or mark it for destruction by the immune system.
Agglutination is one way in which antibodies mark antigens for destruction. Antibodies have at least two sites where an antigen can bind, so they are capable of binding with more than one bacterium or virus. When this happens, the invading particles begin to agglutinate, or form clumps, through a network of antibodies. The clumps eventually become too large to remain in solution in the bloodstream, and precipitate out of solution.
Once the clumps of particles are large enough, they become easy prey for phagocytes — a type of white blood cell that ingests foreign material. Phagocytes ingest and break down the clump, neutralizing the threat of disease. In this way, agglutination enables the body to disarm and remove harmful invading particles.
Hemagglutination, by contrast, is not a naturally occurring process in the body but is instead used to perform assays and testing procedures in molecular biology. Blood type is determined through this process. In blood typing, specific antibodies are added that bind to specific types of red blood cells. If the antibody binds to the red blood cells in a sample, agglutination occurs, and blood type can be confirmed based on which antibody was used.
The concentration of bacteria or viruses in a sample can sometimes be determined using a test called a hemagglutination assay. Certain bacteria and viruses contain compounds that allow them to bind to red blood cells, creating a clump network. In the assay, a diluted sample of virus is added to a diluted blood cell sample, and agglutination is allowed to occur for about 30 minutes. The concentration of the virus can be determined by counting the number of clumps or lattices formed in the mixed sample.