Hematocrit is measured by centrifuging a sample of blood until it separates into plasma and formed elements. The formed elements (mostly erythrocytes, aka red blood cells) sink to the bottom because they are more dense than the plasma. A layer of white blood cells (leukocytes) and platelets (thrombocytes) forms between the plasma and RBCs, and this is called the "buffy coat". (That will be easy to remember because of Buffy the Vampire Slayer haha.)
The alleles that determine blood type are co-dominant, so both the A and B are dominant and the recessive trait results in type O blood. Blood cells make certain antigens which are on the surface of the cell to identify themselves, and they make antibodies found in the plasma to fight off anything foreign.
- Type A cells - A antigens on the surface of the cell, anti-B antibodies
- Type B cells - B antigens on the surface of the cell, anti-A antibodies
- Type AB cells - both A and B antigens on the surface of the cell, no antibodies
- Type O cells - no antigens on surface of cell, both ant-A and anti-B antibodies
Because of the nature of the antigens and antibodies, great care must be taken in blood donations and transfusions so as not to kill the patient due to their plasma antibodies attacking the new blood cells, causing agglutination (clumping of cells). Only certain blood types can be paired up. Obviously a patient can receive the same type of blood they have. But that is not always available. Type O blood is the universal donor because it has no antigens on its surface, so the blood of the recipient has nothing to attack. Apparently, even though type O blood has antibodies against A and B in it, when that blood is donated to an individual, it's not enough to cause a big problem. AB blood is considered the universal recipient because it has antigens for both so anything that gets donated to them will recognize the patient's blood as familiar and no response will take place, and perhaps more importantly, it has no antibodies to attack the incoming foreign blood.
Blood typing is done by taking samples of blood and adding antibodies. If an A antibody is added to a sample and it agglutinates, that indicates it has type A blood (or AB). Type O blood won't agglutinate at all. There is also the Rh factor that gets tested for and that is what gives the + or - to blood types. Here's a chart of the agglutination from this test:
So even when the blood types are compatible, a "crossmatching" is done before any donation/ transfusion to ensure that the blood is compatible. They do this by mixing red blood cells from the donor with plasma from the recipient and make sure no clumping happens.
Okay once again I must say - the human body is AMAZING!!!