What the Heck is "Dosage"?

Residents and other new students in blood banking are often confused by the term "dosage." People that work in blood banks toss around that word all the time.

To be honest, I have been guilty of doing the same thing in my lectures for the Osler pathology course. Judging by the blank looks I get and the questions that people ask me later, I'm guessing that there is a bit of confusion out there about what dosage actually means.

"Dosage, when used by blood bankers, is a word that describes a significant difference in antibody reaction strength depending on the amount of corresponding antigen present on a red blood cell. The difference in antigen quantity is determined by the zygosity of the gene coding for the antigen."

Let me explain.

The intricate details of the genetics of the blood group systems are beyond the scope of this site. However, we can generally say that most of the blood groups show "codominant" expression of antigens. This means that if a particular blood group has two different genetic possibilities (for example, the Kidd blood group system, which has two main genes, called "Jka" and "Jkb"), and a person carries both genes, that person will express BOTH of the antigens. This is illustrated in the table below: