Q. We’re doing immunology right now and I don’t get what the difference is between isotypes and idiotypes. Are they the same thing?

A. Good question! There’s another word that sounds similar too – allotypes. It’s pretty simple – here’s a quick summary.

Antibody isotypes are the same thing as antibody classes. There are 5 major isotypes: IgM, IgD, IgG, IgE, and IgA. The difference between these isotypes lies in the heavy chain (Mu, Delta, Gamma, Epsilon, or Alpha). You can have either kappa or lambda light chains with any of these isotypes. The most plentiful isotype in the body is IgA (if you’re just looking at the blood, the most common isotype is IgG); the least plentiful one is IgE. They all have different functions, which, come to think of it, is a good topic for another post.

Allotypes represent the genetically determined differences in antibodies between people. So you and I both have IgG, but unless we’re closely related, my IgGs are very slightly different than yours – maybe just by a couple amino acids in the constant region of the heavy or light chains. Allotypes are used for paternity testing.

Idiotypes are antibodies that recognize different specific epitopes. The thing that determines the idiotype is way at the end of the variable region; it’s composed of a bunch of different idiotopes (or combining sites).

Of all of these concepts/words, the most important to focus on is isotypes. You should know a little bit (or a lot, depending on the depth of the material in your class) about each isotype.