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Your blood type is determined from the shape and function of your red blood cells. The possible blood types are: A, B, AB and O. However there are several different O types.

Type A: The red blood cells have H-proteins attached around the perimeter which bond with the A sugars in your body. These A sugars are made by your body and your body does not make B sugars.

Type B: The red blood cells have H-proteins on their perimeter which bond with the B sugars in your body. Your body makes B sugars but does not make A sugars.

Type AB: The red blood cells have H-proteins on their perimeter which bond with both A and B sugars in your body. Your body makes both A and B sugars.

There are five different type O scenarios.
For ease of explanation we will list them as 1, 2, 3, 4, and 5 but as far as I know they do not have any such distinction.
1) Type O: The red blood cells have H-proteins on their perimeter but do not bond with A and B sugars. A and B sugars are not made by the body.
2) Type O: The red blood cells do not have H-proteins, A sugars are made by the body but do not bond.
3) Type O: The red blood cells do not have H-proteins, B sugars are made by the body but do not bond.
4) Type O: The red blood cells do not have H-proteins, A and B sugars are made by the body but neither bond
5) Type O: The red blood cells do not have H-proteins, A and B sugars are not made by the body and therefore do not bond.

The Rh factor is also accounted for in the make-up of your blood cells. A person is Rh+ if they have Rh located on the perimeter of the red blood cell and Rh- if they do not.

Another interesting facet of blood types is the way that blood types are passed on through genetics. The A and B types are dominant. This means that if one parent passes you the A type and the other passes you the O type, your blood type will be A, just as getting type B and type O will give you type B. If you are given type A from one parent and type B from the other your blood type will be type AB and two Os will result in type O.

There is however one exception to this rule. Parents also pass on H and h, the big H meaning that your red blood cells will have the H-proteins and the little h meaning that your red blood cells will not have H-proteins. If your parents both pass on to you the little h then you will be a type O no matter what A/B/O they pass on to you. The reason for this is that only type O has red blood cells which do not have H-proteins.