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 confusion out there about what dosage actually means.
"Dosage, when used by blood bankers, is a word that describes a particular quality of an antibody, in which it varies in strength of reaction depending on the amount of antigen present on the surface of a red cell."
— Blood Bank Guy
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 "Jk(a)" and "Jk(b)"), and a person carries both genes, that person will express BOTH of the antigens. This is illustrated in the table below:
| Genotype | Antigens Expressed | |
| Jk(a)Jk(a) | Jk(a) | |
| Jk(a)Jk(b) | Jk(a), Jk(b) | |
| Jk(b)Jk(b) | Jk(b) |
This is different from the way we usually think of genetic expression (the classic dominant and recessive Mendelian genetics), in which a dominant gene suppresses expression of the product of a recessive gene completely.
Since both genes lead to expression of their particular antigens, it stands to reason that a person who carries two copies of a particular gene would have more of the corresponding antigen than someone with only one copy of that gene. For example, the Jk(a)Jk(a) person in the above table would carry more Jk(a) antigen than the Jk(a)Jk(b) person in the next row. Or, to put it yet another way, the homozygous expression of Jk(a) leads to more Jk(a) antigen expression than the heterozygous expression of Jk(a). Does that make sense to you? If not, go back and re-read this from the top, because you won't get the rest of this if you're not clear on what I just said!
So, how does all of this relate to the question we asked at the beginning? Well, many antibodies that people form in response to foreign red cell antigens show no significant difference in strength regardless of whether the cells they are reacting against have antigens coded for by one gene or by two (or, whether the antigens are homozygous or heterozygous). Other antibodies, however, react in a much stronger fashion when targeting a red cell antigen that has homozygous genetics than heterozygous genetics. This phenomenon is called, you guessed it, DOSAGE!
Again, using our example above, if an antibody against the Jk(a) antigen showed dosage, (which it does), you would expect it to react much more powerfully against a red cell with homozygous expression of Jk(a) (the first line in the table above) than against a red cell with heterozygous expression (the second line). There are two blood group antibodies that are really famous for this phenomenon; the Kidd and Duffy blood group antibodies.
In the reference lab, a knowledge of dosage is important in identification of unexpected red cell antibodies, because antibodies showing dosage can be harder to identify unless you have test cells that have homozygous expression of the target antigen. An antibody mixed with cells that have only heterozygous expression may show false negative reactions.
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