What is the difference between positive and negative in blood types?

I understand the differences between O, A, B, and AB. I don't understand what the positive or negative means.
 
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1,202 IKU
Posted on Jun. 25th, 2008
(Edited on Jun. 25th, 2008)

When talking about blood types, the +/- is in reference to the presence of lack thereof of the Rhesus D factor. Similar to the gangliosides that control blood type through surface receptors on RBCs, the Rh(D) factor is also a component of a cell membrane. Blood that is negative (lacks) the Rh(D) factor will develop antibodies against blood containing Rh(D) factor, and those antibodies will attach to and lyse the Rh(D)+ red blood cells (RBCs).

This generally does not present an issue, but can become a complication of pregnancy. The cells of a mother who is Rh(D)- and who has developed antibodies against Rh(D)+ cells will potentially attack the blood cells of a developing fetus, leading to problems shortly after birth, miscarriage, or apparent infertility. A newborn who is Rh(D)+ and has a Rh(D)- mother can get hemolytic disease, meaning that the anti-Rh(D)+ antibodies from the mother are targeting the newborn's blood cells and the newborn's own immune system is killing them. This usually causes jaundice and can lead to heart failure in the newborn.

A mother must first develop the disease-causing antibodies before the newborn will have problems, so not all pregnancies with a (-) mother and a (+) fetus lead to hemolytic disease of the newborn.

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136 IKU
Posted on Jun. 25th, 2008

In responce to the earlier answer on pregnancy complications there is a drug called RhoGAM (see www.rhogam.com). The drug prevents the mother's body from producing antibodies against Rh-positive cells (that the baby may have if the father is Rh-positive, but if the father has recessive Rh-negitive genes it is possible that the baby is Rh-negitive as well). It is administered to the mother halfway through the pregnancy and once again when the baby is born if the baby is Rh-positive. If the baby is Rh-negitive then there is no need for the second injection. The greatest risk is to the second baby if the first is Rh-positive and the mother produces antibodies against Rh-positive cells and the second baby is Rh-positive. The drug must be used in the second pregnancy as contact with the first baby's blood during birth increases the risk of Rh-antibodies being produced, however if the drug is used in the first pregnancy and after birth the second pregnancy should be exactly the same as the first.

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1,292 IKU
Posted on Jun. 25th, 2008

Positive and negative refers to a person either having or not having the antigen RhD. If positive, the person does, and if negative they do not.

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106 IKU
Posted on Jun. 25th, 2008

positive and negative refer to the presence of the Rh factor in blood

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