sciencenote: As you may know, there are four...

Saturday, March 5, 2011

As you may know, there are four blood types: A, B, AB, and O. The difference between them is that the red blood cells in each blood type is covered in a different protein. These proteins are actually modified with some sugar attachments, and there are two types of them: the A-type proteins and the B-type proteins. If you have only A-type proteins, you are A-type blood type. If you have only B-type proteins, you are B-type blood. If you have both A- and B-type proteins, you are AB, and if you have neither, you are O-type.

This system is a bit more complicated because of how the immune system works. You see, the immune system knows what’s really part of you and what’s not part of you. When it comes across something new, it assumes it’s an invader (like a virus) and raises an army that recognizes this foreign object to kill it. That’s all good – most of the time. In immunology lingo, anything the immune system recognizes (whether it’s part of you or foreign) is called an antigen. The tools the immune system uses to attack foreigners aree called antibodies.

In the ABO blood types, if you have an A-type, the immune system earlt in life recognizes that these A-type proteins as part of you, and so it leaves them alone. To rephrase, the A-type antigens are not attacked by the immune system. Likewise, if you have B-type blood, your immune system recognizes the B antigens as part of you and leaves them alone. In AB-type blood, both are recognized as friendly, and because there are no A- or B-type antigens in O-type blood, both are recognized as foreign.

In biological terms, a foreign chemical causes the creation of antibodies against it. So in A-type blood, there anti-B antibodies because the immune system recognizes B-type red blood cells as foreign. In B-type blood, there anti-A antibodies. In AB-type, there are no antibodies against either A- or B-type antigens. In O-type blood, there are antibodies against both A- and B-type antigens.

The presence of these antibodies is why in blood transfusions, O-type people cannot accept any other type of blood except O. Also, that’s why AB blood can accept from any blood group. The table below from Wikipedia sums it up nicely.