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Agglutinin is a substance in the blood that binds individual molecules together, forming a large mass. It is part of the immune system's response to foreign bodies, such as bacteria and viruses, recognizing and rapidly binding to the proteins on an invading organism. The interaction is similar to the antibody-antigen relationship. Some types of agglutinin are in fact antibodies while other agglutinin substances are lectins, a type of protein that readily binds sugars.
When agglutinins are released in a suspension, they bind to specific particles and coalesce them into a single mass. Afterward, the mass sinks to the bottom of the suspension, resulting in a clear fluid. This process, known as agglutination, can be used to identify the cause of an infection. It can also be used to identify an individual's blood type.
One of the jobs agglutinins perform is preventing foreign blood types from entering the blood stream. People with type A blood, for example, have agglutinin B in their blood to destroy type B blood cells. Similarly, if a person's blood type is B, agglutinin A is present in the blood to destroy type A blood cells. A person with type O blood will have both agglutinin A and B in their blood to prevent type A and type B blood cells from entering the blood stream. Individuals with type AB blood don't have either agglutinin in their blood.
When the body is exposed to cold temperatures, cold agglutinins bind to red blood cells and force them to clump together. Normally, there are low amounts of cold agglutinins in the blood. During an infection, the level of cold agglutinins rises, leading to problems such as pale skin and numbness in the hands and feet. As the skin warms up, the symptoms generally go away. In extreme circumstances, blood clumps can block vessels that supply blood to the ears, nose, finger tips and toes, leading to tissue damage that is similar to frostbite. In some cases the tissue damage can lead to gangrene.
Typically, the type of infection causing elevated cold agglutinin levels can be determined. This is done by measuring the amount of cold agglutinins in the blood after a series of dilutions. In a healthy person, the final dilution generally has undetectable levels of cold agglutinins — a ratio of about 1 to 40. Higher dilutions generally indicate conditions such as pneumonia, infectious mononucleosis or hepatitis C. Extremely high dilutions, such as 1 to 1,000, could indicate the presence a more serious condition, such as lymphoma.