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What Is H Antigen?

The H antigen is a foundational substance on red blood cells, determining blood types within the ABO system. It's a precursor to A and B antigens; without it, type O blood exists. This molecular building block is crucial for compatibility in blood transfusions and understanding genetic inheritance. Curious about how this impacts your health? Let's delve deeper into the world of blood types.
Jillian O Keeffe
Jillian O Keeffe

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.

Scientist with beakers
Scientist with beakers

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.

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Discussion Comments


Don’t forget there are also people with AB blood type who can receive blood from anyone. Also, all blood can have or not have a protein called the Rh factor which affects who a can donate blood to who. Simply put, if your blood already has an antigen or protein, it doesn't hurt to receive it in a transfusion but if you don’t already have it then it can be very harmful.


Blood transfusions work by first identifying the receiver’s blood type. If a person has no A or B antigens (type O blood) then you cannot transfuse blood from donors with type A, B, or AB blood. If this happens, the receiver’s body sees the A and B antigens as a foreign object and rejects the new blood. To make things simple let’s use an analogy and say that everyone has an apple. Here the apple represents the h antigen in everyone’s blood. But some people’s apples have a candy or caramel coating which represents the A and B antigen. Now let’s pretend that everyone with a normal apple (O blood type) is also allergic to candy and caramel coating. So if you gave someone with O blood type a candy or caramel apple their body could not accept it. They need to receive blood from someone with the same blood type. The same is true for people with A and B. The can receive any normal apple (O blood type) and any apple that is the same as their own but cannot receive A type if they are B type and B type if they are A.


So if most people have a different blood antigen than how do blood transfusions work?

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      Scientist with beakers