Red blood cells have proteins called antigens on their surfaces. A person with A antigens is said to have Type A blood. Someone with the B antigen has type B blood. If you have both the A and the B antigens on the surface of your red blood cells, then your blood type is AB. Someone who does not have either of those antigens is said to be type O.

If type A persons receive blood from a type B or AB person, they will develop antibodies to the B antigen which would be seen as a foreign body. Those antibodies would attack the red cells of the donor's cells in the recipient's system and cause a major transfusion reaction.

If type B patients receive blood from a type AB or A donor, they would develop antibodies against the A antigen, which would be seen as a foreign body. Those antibodies would attack the red cells of the donor circulating in the recipient's system and cause a major transfusion reaction.

If type O individuals receive blood from a type A, B, or AB donor, they would develop antibodies against the A and B antigen, which are both seen as foreign bodies. Those antibodies would attack the red cells of the donor's blood circulating in the recipient's system. A type O individual can only receive blood from another type O person.

Since type O persons do not possess either the A or the B antigen, it is safe to transfuse type O blood into an A,B, or AB recipient. There are no foreign bodies on the O red cell which would stimulate antibody production in the recipient. This is why type O people are called the Universal Donors.

What determines your blood type? The antigens from your parents are determined by alleles, a form of genetic information. You get one from each of your parents. For example, if you inherit an A allele from your mother and a B allele from your father, your blood type would be AB.

If you inherit an A allele from your mother and an A allele from your father, your alleles' genotype would be AA, which would be called type A blood.

If you inherit an A allele from one parent, and the other parent is type O (no A or B antigen at all), then your alleles' genotype would be AO, which would be called type A blood.

If you inherit a B allele from one parent and the other parent is O, your alleles' genotype would be BO, resulting in what is called type B blood.
You would have to get an O antigen from each of your parents to have OO as your alleles' genotype, to have type O blood.

The A and B antigens are the major antigens of concern. But there are other important antigens on some red cells also. One of these is called the Rh factor. About 85% of Caucasians and about 95% of African-Americans possess the Rh antigen on their red cells. These people are called Rh Positive. For example, if a type A person has the Rh antigen, he/she is said to be A Positive.

If an AB individual does not have the Rh antigen, he/she is said to be AB negative. An Rh negative person cannot receive blood from an Rh Positive donor, because that person would form antibodies to the Rh factor which would be seen as a foreign body circulating in the recipient's system. Those antibodies would destroy the red blood cells from the donor, causing a major transfusion reaction in the recipient.

O Negative individuals are the true Universal Donors. They don't have the A, B, or Rh antigens on their cells, therefore O Negative blood can be given to anyone without antibodies to those factors being formed. But they can only receive O Negative blood.

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After working 20 years in a medical laboratory as an ASCP-certified Med Tech, I searched for my life's purpose and switched careers to teach high school science. I taught at-risk students in an inner-city school. After retiring, I started writing and wrote a memoir about the joys of teaching and the obstacles I fought. No Child Left Behind? The True Story of a Teacher's Quest by Elizabeth Blake on Amazon.com and BarnesandNoble.com Drugs, gangs, a riot, shootings, murdered students, abusive principals - all contributed to the stress which eventually drove me from the students I loved. The book explains the lessons I learned while overcoming the stress and celebrates triumph over adversity. To read the first page and reviews, please visit http://www.elizabethblakeonline.com