Why are there Four Human Blood Types?

Blood type differs among individuals much as body type, hair color and eye color do. We get our blood types from our parents and grandparents (an inherited trait). In simple terms, different types of blood are caused by molecules called antigens. These are the key factor in starting the immune response at the blood level. The basic task is to protect against antigens that aren’t naturally in a person’s body naturally.

The four major types are the commonly known ABO system. For general purposes, we can separate an “A” blood type from the “B” type, for example. Mixing one with the other can cause life-threatening immune responses in the body. The four types are distinguished by the presence or absence of “A” antigens and “B” antigens. The red blood cells in our systems contain either “A” antigen, “B” antigen, both “A” and “B” or none at all.

With this information, doctors and laboratory personnel classify blood types as A, B, AB or O. If you have type A blood, you also have anti-B molecules. Type B blood has anti-A molecules. Type AB has neither of these antibodies and people with Type O blood has anti-A and anti-B molecules that work against both types. This is the basic reason that a person in need of a blood transfusion cannot receive blood from someone who has an antigen that their blood does not contain.

If this occurs by mistake the receiving body will start defending against the “foreign” substance. It’s best, of course, to conduct a transfusion with exactly the same type of blood. If this is not possible, a person with Type O blood can donate to someone with Type A blood, for example, because Type O doesn’t have either of the antigens that the receiving body would defend against. This means that people with Type O blood can only receive a transfusion from another Type O person. Doctors cannot put “A” or “B” antigens into the bloodstream where these are not already present.

While most people are familiar with the four major blood types, research shows that there are more than two dozen distinct blood groups and hundreds of different antigens that make one type slightly different from another.

As mentioned earlier, these four basic groups are only the beginning. Research through the decades has uncovered subcategories based on the Rhesus system. Some blood groups have a “D” antigen and some don’t. This leads to RhD negative and RhD positive blood. This is where the term A+, A-, B+, B-, O+ and O- come from. In the same way that it is harmful to mix A blood with O blood, RhD negative people should not receive RhD positive blood.

There are a number of specific groups beyond even these common types. Doctors and research scientists have known for years that mixing these blood types causing life-threatening “clumping” of the blood cells. The formal term for this is agglutination. The man who discovered this system received the Nobel Prize for his work.