Q. I have a question about blood typing. I understand that in forward typing, we use anti-A and anti-B antibodies. I was wondering how you know which antibody caused the agglutination, and how do you find out what type of blood is in there. Do you do two forward typing reactions, first with Anti-A then with Anti-B?

A. You’re exactly right – you do two reactions: one with anti-A and one with anti-B!

When you do forward typing, you use antibodies that you buy from a company. So you have a little tube of anti-A antibodies and a little tube of anti-B antibodies. You take one test tube, drop a few drops of the anti-A antibody reagent in there, and add a drop or two of the patient’s blood. If you see clumping, you know the patient’s blood must have A antigen on the red cell surface. You repeat the same procedure in a new test tube, this time with the anti-B antibody reagent.

Usually you do the forward typing first, just by convention, but you wouldn’t have to. Both the forward and reverse typing need to be done and they need to match up (if the patient has type A blood on forward typing, he/she should have anti-B antibodies on reverse typing). The order in which you perform the tests doesn’t really matter.