Permit me to introduce myself. My name is Major Histocompatibility Complex. I am a protein and a strange paradox, both fighter and matchmaker. I am on duty day and night, 365, working as the second line of defense against pesky and sometimes deadly viruses. I am also a player, not in the naughty sense, in the immune response to autoimmune, bacterial, and parasitic infections. Because I am the amazing protein that I am, my very presence elicits male and female human sexual attraction. I doubt there is any other protein that can claim to be both a protector and a matchmaker.
When cells break down old proteins, they place the peptide fragments on the outside of the cell surface. I pick up the fragments and display them for the immune system to inspect. If the fragments are normal, the immune system ignores them. If the fragments are abnormal, however, the immune system kills the cell. 
When certain foreign invaders activate me, such as poison ivy, I tend to cause a rather dramatic reaction. You will notice this reaction as a bright red, itchy rash. For that I apologize. On a more positive note, if I come across a dangerous mutation, such as cancer cells, I recruit my troops to come and destroy them. Now on to my role as cupid. Research studies demonstrate that men prefer the t-shirts of women with MHC scent dissimilarity. On the other hand, women prefer the scent of MHC heterozygosity and show no preference for dissimilarity. Since the majority of people are not aware of my existence or importance in the immune response, I believe I can move into a higher place of significance by marketing my attributes in human sexual attraction.
Sources:
http://beheco.oxfordjournals.org/content/14/5/668.abstract?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=major+histocompatibility+complex&andorexactfulltext=and&searchid=1&FIRSTINDEX=10&resourcetype=HWCIT