# Blood Types --- **Blood types** are a feature of an individual's [[red blood cells]] that act as a sort of makeshift [[MHC molecule#MHC I|self-identifier]] to the body. Since RBCs are so small and dumb they only have a little glycoprotein and maybe another little molecule (rhesus factor) to indicate they belong to the organism. Humans only have three different types of glycoproteins, someone either has the Rh factor or not. This comes in handy because they are easily transferred from one person to another *as long as the types are compatible*. Type A blood indicates the presence of an A glycoprotein. Type B blood indicates the presence of a B glycoprotein. Type AB blood indicates the presence of both A *and* B glycoproteins. Type O blood indicates that neither A *nor* B are present. A and B alleles are [[dominance and recession|codominant]] with each other which is how you can get an AB type. Both A and B are [[dominance and recession|completely dominant]] to O, which is why you never see AO or BO, and the only way you can get O is if both alleles are O. ![[blood type ABO.png]] >[!biology]- Genotypes There are 6 possible [[genotype and phenotype|genotypes]] for blood types in humans. The allele uses "I" or "i" (as in "ice") to indicate if the gene is dominant or recessive, and the superscript "A" or "B" to indicate the type of glycoprotein. > ![[blood type squares.png]] **Rh factor**, or Rhesus factor, is a specific protein only found in the RBCs of some people. If you have these proteins you are "Rh positive" and if you do not you are "Rh negative". This is much more straightforward than the ABO. If you have the Rh factor your cells will ignore any other Rh factor it sees. If your cells do not have Rh factor, your cells will attack anything with Rh factor that it sees. Rh factor mismatch can cause a transfusion reaction of course, but one specific problem that can arise is the [[hemolytic disease of the newborn]] where an Rh- mother gives birth to an Rh+ baby. Although the pregnancy will typically be okay, during birth the mom will see the Rh+ blood and make memory cells about it, which will then attack any subsequent Rh+ babies. One interesting thing is that this does not mean that these are spread out through the population equally—you might think an O type is quite rare, but in fact O+ is the most common blood type in America. ![[blood types in america.png]] ## Compatibility In order for a blood transfusion to work you need to make sure the types are compatible. if you do not have A or B glycoproteins, you *will* have [[antibodies]] that will attach to them. For example, if you are type A, you have "anti-B" antibodies, and vice versa. (If you are type O you have anti-A *and* anti-B antibodies.) This is important because giving someone the wrong type of blood infusion will result in a [[blood transfusion|transfusion reaction]]. ![[blood type compatibility.png]] As you can see from the chart above, O type blood can be given to anyone and AB type blood-havers can take blood from anyone. If you factor in Rh factor, since Rh- can be donated to Rh+, but not vice versa. So, ultimately O- blood is the **universal donor** and AB+ blood havers are the **universal recipients**. ___