| MadSci Network: Immunology |
Hello,
You ask some interesting questions, and I'll try to handle them one at a time.
Q: “Why do people with Type A blood have Anti-B antibodies?”
The ABO blood typing system is used to describe the presence of carbohydrate modifications of proteins on red blood cells. If a person has type A blood, then some of their carbohydrates end in N-acetylgalactosamine, and if a person is type B, their carbohydrates end in galactose. Type O antigens are one sugar shorter, and thus have an "empty" spot where the last sugar would be in A or B blood.
Antibodies that bind a certain “blood type” must recognize the carbohydrate structures described above. It turns out that the bacteria that normally live in human intestines have antigens that are identical, or extremely similar, to the blood group antigens. People make antibodies specific to the carbohydrates on the bacteria, and these antibodies cross-react to blood group antigens. This is why there are antibodies reactive against other blood group antigens, even if the subject has never been exposed to other types of blood.
Q: “Is this evidence that all humans are related?”
I don't think so.
Q: “I don't believe that humans are born with the ability to recognize specific disease antigens, why is blood type any different?”
As with all things in Immunology, beware of generalized statments. Humans are born with the ability to recognize specific disease antigens. It’s just that almost all of a newborn’s cells are naive, so it takes some time to expand the repertoire of responding cells. Neonates do have very low antibody titers, but these will go up as the number of B cells expands with age, and as the first infections are resolved. Blood group reactivity is not different from other situations; it develops early because the gut flora provide an early and constant source of antigen against which the immune system can react.
I hope this answers your questions. If you have more questions, please email me directly, or submit to MadSci Network.
Dave
Reference: Fundamental Immunology, third edition, edited by Bill Paul