Sentence:  <WSD>Allotypes</WSD> were originally defined by antisera that differentiated allelic variants of immunoglobulin (Ig) subclasses. The <WSD>allotype</WSD> is due to the existence of different alleles at the genetic locus that determines the expression of a given determinant. 
Meaning of the word marked with <WSD> tags:  An immunoglobulin allotype; the allele of the antibody chains found in an individual.