Apparently we have some geeks in the crowd...

Info below supplied by Don (original source: http://waynesword.palomar.edu/aniblood.htm):

Since Rh negative people may produce anti-Rh antibodies, Rh positive blood should not be given to an Rh negative recipient. Based upon the above table, Rh positive recipients can theoretically receive positive or negative blood, and Rh negative donors can theoretically give to Rh positive and Rh negative recipients. Therefore, the "universal donor" is O Negative, while the "universal recipient" is AB Positive.

Anti-Rh (immune-type) antibodies can readily pass through the placental capillary membranes. A serious potential problem called maternal-fetal blood incompatibility or Rh Disease could occur with a pregnant Rh negative mother who carries an Rh positive fetus. Leakage of fetal red blood cells (RBCs) into the mother's system through minute lesions in the placenta may cause her to produce anti-Rh antibodies. This could occur during the latter months of pregnancy or when the baby is delivered. Because of the time interval involved in producing a concentration (titre) of antibodies, the first Rh positive child may not be adversely affected. However, a subsequent Rh positive child may be at risk because the mother's anti-Rh can pass through the placenta, thus entering the fetal circulatory system and clumping fetal RBCs.

The medical term for this maternal-fetal condition is "erythroblastosis fetalis" because of the presence of nucleated, immature RBCs called erythrobasts in the fetal circulatory system. The fetus bone marrow releases immature erythroblasts because of the destruction of mature RBCs (erythrocytes) by the mother's anti-Rh antibodies. RhoGam®, a serum containing anti-Rh antibodies, is now given to Rh negative woman within 72 hours after giving birth to their Rh positive baby. The RhoGam® enters the mother's circulatory system and destroys any residual fetal positive RBCs that may be present in her system. This prevents her from producing anti-Rh antibodies. RhoGam® must be given after each Rh positive baby. In this scenario of erythroblastosis fetalis, the fetus must be Rh positive, the mother Rh negative and the father Rh positive. You can easily determine the exact genotype of the mother and fetus, but the father's genotype could be homozygous or heterozygous Rh positive.

Rh incompatibility is summarized in the following table:

Rh Pos Father + + or + - X Rh Neg Mother - - 1st Rh Pos Child + - Rh positive RBCs from the fetus enter the mother's circulatory system. After several days, the mother begins to produce anti-Rh antibodies. 2nd Rh Pos Child + - Anti-Rh antibodies from mother pass through placenta and enter fetal circulatory system. The antibodies begin clumping fetal positive RBCs.

There are also reported cases of maternal-fetal blood incompatibility with the A-B-O blood groups; however, the Rh factor appears to be much more common. The larger anti-A and anti-B antibodies (IgM type) with molecular weights of 950,000, apparently don't penetrate the placental membranes as readily. In the case of A-B-O blood incompatibility, the anti-A and anti-B antibodies of a type O mother may enter the circulatory system of a Type A or Type B fetus, thus causing agglutination of the fetal RBCs. If the fetal blood cells just happened to be Rh positive and entered the mother's circulatory system, they would be destroyed by the mother's anti-A or anti-B antibodies before her system began to produce anti-Rh immune-type (IgG) antibodies. In this latter case, the anti-A or anti-B antibodies would actually serve as a natural immunity to Rh maternal-fetal blood incompatibility.

Like most topics in biology, the true life explanation is a lot more complicated. Rh inheritance is no exception. It actually involves three different pairs of genes at three different loci on homologous chromosome pair #1. The gene pairs are C & c, D & d and E & e. The terms "positive" and "negative" essentially refer to the D factor, so homozygous DD and heterozygous Dd are positive, while homozygous recessive dd is negative.

There are some good reasons for a husband to not donate Blood to his wife during her childbearing years. During this time, a women who plans to become pregnant, receiving Blood from her husband may pose a small risk to the infants born of these pregnancies. If, after the Blood transfusion the woman develops an antibody to an antigen on the father's red Blood cells, and the subsequently born fetus inherits the father's red cell antigen, the antibody from the mother may enter the Bloodstream of the fetus causing destruction of fetal red Blood cells. This may cause serious anemia in the fetus and excessive jaundice in the infant after birth. This is a known major cause of brain damage. Special Blood transfusions, using selected red Blood cells that do not have the particular in-compatible and offending antigen, are available when this condition is pre-diagnosed. Of course, we suggest autologous Blood donation for the mother. However, for those mothers who are unable to make an autologous donation, the decision to select her husband as a donor should always take this risk under consideration, and specific consultation with your pediatrician on this subject is essential.

BLOOD TYPE COMPATIBILITY CHART **
Blood Type of Recipient	Donor Can Be for.....
	Red Cells	Whole Blood	Plasma
O +	O +; O -	O +; O -	any O; A; B or AB
O -	O -	O -	any O; A; B or AB
A +	any A +; A -; O + or O -	any A + or A -	any A or AB
A -	any A - or O -	A -	any A or AB
B +	any B +; B -; O + or O -	any B + or B -	any B or AB
B -	any B - or O -	B -	any B or AB
AB +	any AB +; AB -; A +; A -; B +; B -; O +; or O-	any AB + or AB -	any AB
AB -	any AB -; A -; B -, or O -	AB -	AB