What’s the difference between blood in people that means we have different “types” and why are some incompatible?
In: Biology
In the strictest sense, a blood type refers to the presence of certain types of *antigens* on the surface of blood cells.
Antigens are molecules such as proteins or carbohydrates that can trigger a response by the immune system, such as those found on the surface of certain viruses and bacteria, or proteins on the surface of pollen grains.
In general antigens produced by your body’s own cells will not produce an immune response by your respective immune cells. There are mechanisms to prevent this from happening. In rare cases where this does occur, it can produce diseases like rheumatoid arthritis or lupus.
However, different people may produce different antigens on their blood cells, because they inherited a given set of genes from their parents that produces that antigen, but not another one.
This means that if blood from one type is transfused to someone with a conflicting one, it may cause proteins called antibodies to attach to the antigens on the surface of blood cells. This causes blood cells to clump together, which can result in lethal blood clots.
There are in fact 49 different Blood Group Systems known at present, but in most of those, variation is very rare and nearly everybody has one type and only a few thousand differ. This makes adverse reactions unlikely with those types.
The most important types are the “ABO” system and the “Rh” system.
These are the types of antigens where there is the greatest degree of variation in the human population.
In the ABO there is a set of two genes that produce a kind of carbohydrate on the surface of cells. One called A, one called B.
Depending on your parent’s blood types you may get the gene for A, B, both A and B. You may also receive a non-functional gene that doesn’t produce an antigen. If you get two copies if this gene, neither is produced and your blood type is O, which is the rarest.
Unfortunately, because either of these antigens is common in other people, if you only have B for example, your body is likely to develop an immune response to A due to being around other people in the same way that exposure to a virus causes an immune response. This is how vaccines work. So, just interacting with other people essentially vaccinates you against A as a young child.
Because of this the immune system produces antibody proteins that bind strongly to the A antigen.
This will result in a life threatening reaction if you receive A type blood, but not if you receive O.
In the case of having O, you cannot be given other blood types because it’s likely your body produces antibodies that react with both A and B.
The Rh system is more complex, but works by the same basic kind of mechanism.
In general, people will only be given the exact same blood type except in an emergency because adverse effects can still occur otherwise.
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