id·i·o·type(d–tp)
n.
A set of one or more antigenic determinants specific to the variable region of an immunoglobulin molecule.
The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2003. Published by Houghton Mifflin Company. All rights reserved.
http://pathmicro.med.sc.edu/book/immunol-sta.htm
I don’t know how good the above online text is, but the price is right (free). You can look up the basics of immunology there.