Antigens are molecules that immune systems can recognize as foreign and mark for destruction. The molecules themselves can have long and complicated scientific names, so antigens may be given short names for easy recall. Sometimes, the antigens are simply given letters to distinguish them from other molecules, and this may result in different antigens having the same name. This is the case for H antigen, which can either refer to a human blood antigen or a bacterial antigen.
The human H antigen forms part of the ABO blood system. This system of blood typing separates people into A, B, AB, or O blood types, judging by the type of antigen the person has on the surface of the red blood cells. Someone with A-type blood has red blood cells with only the A antigen, for example, and someone with O-type blood produces neither A nor B antigen.
H antigen is a molecule that is present on most people's red blood cells. It is a building block that both the A and B antigens are formed from. If someone has genes for A, B, or both, then enzymes work to finish off the raw material of the H antigens to make a new A or B antigen on the surface of the cell. A antigens are produced by an enzyme adding an N-acetyl galactosamine molecule onto the H antigen, and enzymes add a D-galactose onto the H antigen to make a B antigen.
People with O-type blood do not produce A or B antigens. Therefore, the H antigens present on the surface of the red blood cell remain unaltered. This is the case for the majority of O-type people in the world.
Blood types are generally traceable through genetic inheritance of A, B, or O genes 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 ineffective genes for the H molecule. If the H antigen is not produced, then the A and B gene products 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 needs to recognize bacteria as they are the cause of many infections, and so many parts of bacteria are antigenic. The H antigen refers to the flagella, a propeller-like structure that many bacteria use to move around.
Different bacteria have flagellae that are made up of different proteins and, therefore, have different H types. Even one species can produce different flagellar antigens that may help avoid detection by the immune system. One such bacterium is Salmonella typhimurium, which can produce two distinct H types.
The H type of a bacterium may also be useful in identification. The bacterium Escherichia coli can have about 50 different H types. One such type is the food-poisoning bacterium E. coli 0157:H7, which has the seventh type of H flagellar antigen of that species.