MCQ Obstetrics Answer 8

The direct Coomb's test is used to detect maternal antibodies attached to fetal cells in haemolytic disease of the new-born. Usually it is "big D" we are dealing with. The baby's Rh positive cells have maternal anti-D attached. As she will have been sensitised some time before, this anti-D will be IgG. (The early response to an antigenic stimulus is the production of IgM, which is replaced in time by IgG.) Antibodies (raised in sheep, I believe) against this maternal IgG, will attach to affected fetal cells and can be used to produce agglutination. This is a positive test.

The indirect test is used to detect antibodies in the mother's blood during pregnancy. Let us say that she is Rh negative with anti-D antibodies. Rh positive cells are introduced into a sample of her serum. They will be coated with her anti-D. To this mix is added antibodies to anti-D and an agglutination reaction takes place. (Talk about big fleas having little fleas!)

The test can be used with appropriate cells and antibodies to check for other antibodies and by running the test with different dilutions of maternal serum, a measure of the amount of antibody present can be obtained.

The curious among you may wonder why anti-D on its own does not cause agglutination. Apparently it is an incomplete antibody. What is an incomplete antibody? The books say it is one, which does not cause agglutination under such circumstances!

I hope you feel much better for knowing all that.

Category: Obstetrics MCQs