Check out this interesting concept. You already know that the MHC I receptor is present on (pretty much) every cell in the body and presents antigen to cytotoxic T cells, and that the MHC II receptor is present only on specialized antigen-presenting cells and presents antigen to helper T cells. Right. But did you know that these same receptors may be helping a woman decide on a mate? It seems that women prefer mates with MHCs very different from their own. That seems like a good idea – it helps provide diversity within the genome. It would be hard to get a sample from every guy and bring it to the HLA-typing lab, but no need: the brain can detect MHC differences by smell (they’re associated with pheromones)! Weird. And cool.