Expressed major histocompatibility complex class II loci in fishes

Peptides derived From parasites are presented to T helper cells by major hi stocompatibility complex (MHC) class II alpha beta heterodimeric cell-surfa ce molecules. In mice and humans, the genes encoding these antigen-presenti ng molecules are known to be polymorphic and polygenic. Multiple loci for M HC class II A and B genes are proposed to allow for an increased peptide-bi nding repertoire. The multigenic nature of expressed MHC class II loci and the differences between these loci in fishes are the focus of this review. Particular emphasis is placed on an evolutionary comparison of class ii B l oci, especially two class II B loci that have undergone dramatic changes fr om one another suggesting an ancestral gene duplication event that took pla ce at an early stage in the evolution of teleosts. The number of functional class II alpha beta heterodimers may have a profound impact on the organis ms ability to battle constantly evolving parasitic infections.