Each MHC protein binds to a specific peptide sequence, yielding a set of uniquely bound peptide-MHC complexes for each individual.
During cellular turnover, the MHC-peptide complex is shed from the cell surface and the fragments are dispensed in bodily fluids such as blood serum, saliva, and urine.
Unfortunately, the MHC-heterozygote advantage hypothesis has not been adequately tested.
The second hypothesis for the maintenance of MHC diversity by parasites is the Red Queen hypothesis.
The extreme diversity in the MHC would cause individuals sharing MHC alleles to be more likely to be related.
Major histocompatibility complex genes, which control the immune response and effective resistance against pathogens, have been able to maintain an extremely high level of allelic diversity throughout time and throughout different populations.
Humans can not only detect, but also assess, and respond to environmental (chemical) olfactory cues—especially those used to evoke behavioral and sexual responses from other individuals, also known as pheromones.
Pheromones function to communicate one's species, sex, and perhaps most importantly one's genetic identity.
One is that there is selection for the maintenance of a highly diverse set of MHC genes if MHC heterozygotes are more resistant to parasites than homozygotes—this is called heterozygote advantage.
The second is that there is selection that undergoes a frequency-dependent cycle—and is called the Red Queen hypothesis.
The vast source of genetic variation affecting an organism's fitness stems from the co-evolutionary arms race between hosts and parasites.