Saturday, December 03, 2011

Blood bank lesson: blood group antigens

I've been at the new job for a week now, and I'm thinking it's going to work out. I've been asked what exactly I'll be doing there, and I would like to explain, but first you'll need a blood bank lesson or two.

You've got stuff on your red cells. They're little nubbins of proteins and sugars, and we blood bank types call them antigens. They're genetically determined and you inherit them from your parents, so what's on your red blood cells will be a mix of what's on your Mom's and on your Dad's. You're probably familiar with the A and B antigens, since they determine blood type, and the D antigen is the one that makes you "Rh positive". Those were the first ones discovered, a long time ago, and since then there have been dozens more, if not hundreds, added to the list. Scientists, having a deep-rooted need to sort, classify, and name everything, have sorted, classified, and named them all. For some people, these antigens become an issue.

When you get a blood transfusion, you're exposed to a mix of antigens from the donor's cells, and your immune system might decide to make antibodies against the ones it's not familiar with. The same thing happens in pregnancy, because the baby's cells will have some antigens from the father's side, which the mother's body has never seen. There's no guarantee that you will develop an antibody if you're exposed to something new, but once you've made an antibody, we have a problem.

Let's say you get a blood transfusion from someone who's got the Kell antigen on their cells. It's a common one - about 90% of people have that one. If you're in the 10% of people who are Kell-negative, you might make an antibody in response to your exposure. Next time you go to the hospital and need a transfusion, the blood bank will find the antibody in your blood when they do a "type and screen". Once they've done their investigative magic and figured out the identity of your antibody, they will have to find you some blood that doesn't have the corresponding antigen on it, because now that you've got those antibodies in your system, if you see the Kell antigen again, your antibodies will destroy those transfused cells and all the red cell guts will be free in your blood and that's a bad thing.

How do they find the Kell-negative blood for you? With antibodies! There are commercial preparations of purified Anti-Kell (and anti-pretty-much-everything) that the blood bank can buy and use to test blood units. So they'd take a little drop of cells from the donor units, and mix them with this antibody solution, and see if the cells clump up. If they do, the cells are positive for the Kell antigen, and you can't have those. The ones that don't clump up are safe for you, because if this Anti-Kell solution doesn't clump up the cells and destroy them, neither will the Anti-Kell you've got floating around in your plasma (the liquid part of your blood).

Recap: Blood group antigens are on your cells. When you're exposed to a foreign antigen, you can make an antibody that will remain in your plasma, which can make subsequent exposures dangerous. Blood bankers use antibody solutions to test blood for specific antigens, when necessary, to be sure to avoid that situation.

Questions from the class?

No comments:

Post a Comment