Antigens can be given short names for easy recall, but this can result in different antigens having the same name. The H antigen can refer to a human blood antigen or a bacterial antigen. Human H antigen is part of the ABO blood system and is a building block for A and B antigens. The H antigen is also found in some bacteria and can be useful for identification.
Antigens are molecules that the immune system can recognize as foreign and mark for destruction. The molecules themselves can have long and complicated scientific names, so the antigens can be given short names for easy recall. Sometimes, antigens are simply given letters to distinguish them from other molecules and this can result in different antigens having the same name. This is the case with the H antigen, which can refer to a human blood antigen or a bacterial antigen.
Human H antigen is part of the ABO blood system. This blood typing system separates people into A, B, AB, or O blood types, judging by the type of antigen the person has on the surface of their red blood cells. Someone with type A blood has red blood cells with only A antigen, for example, and someone with type O blood produces neither A nor B antigen.
The H antigen is a molecule found on the red blood cells of most people. It is a building block from which both the A and B antigens are formed. If someone has the genes for A, B or both, then enzymes work to finish the raw material of the H antigens to create a new A or B antigen on the surface of the cell. The A antigens are produced by an enzyme that adds a molecule of N-acetyl galactosamine to the H antigen, and the enzymes add a D-galactose to the H antigen to form a B antigen.
People with type O blood do not produce A or B antigens. Therefore, the H antigens present on the surface of red blood cells remain unaffected. This is the case for most type O people in the world.
Blood types are usually traceable through the genetic inheritance of genes A, B or O from parents. Usually, someone with one parent with only A genes and another with only B genes will have AB blood. This system does not apply if the person inherits two genes ineffective for the H molecule. If the H antigen is not produced, the products of the A and B genes have nothing to work with and the person ends up with an O blood type. This rare occurrence is known as Bombay phenotype as the blood type was first found in Bombay, India.
An alternative type of H antigen can be found in some bacteria. An immune system must recognize bacteria as they are the cause of many infections and many parts of the bacteria are antigenic. The H antigen refers to the flagellum, a propeller-like structure that many bacteria use to move around.
Different bacteria have flagella made up of different proteins and, therefore, have different types of H. Even one species can produce different flagellar antigens which can help avoid detection by the immune system. One such bacteria is Salmonella typhimurium, which can produce two distinct H-types.
The H type of a bacterium can also be useful for identification. The bacterium Escherichia coli can have about 50 different types of H. One such type is the food poisoning bacterium E. coli 0157:H7, which has the seventh type of H flagellar antigen of that species.
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