For this reason the sites limit their DNA analysis to the genes of the immune system to make its matches.

These genes, known as HLA (human leukocyte antigen) genes (also sometimes referred to as MHC for major histocompatibility complex) control how the immune system recognizes and fights off viruses, fungi and bacteria and is also the portion of the genome doctors look at when looking for compatible organ donors.

For example, the research on couples and immune systems does not tell us how big of a factor this actually plays in mate selection.

The tube is then sent to a lab, where it is tested for the presence of two genetic markers.

The two markers tested for are the serotonin uptake controller, which is involved in how people handle positive and negative emotions.

If you’re looking for love online you can forget listing your hobbies and favorite books.

Dating sites can now find your perfect match based on DNA.

How it works: You get an Instant Chemistry DNA kit in the mail when you sign up for a Singld Out membership. You spit in the tube, mail it to Instant Chemistry and get results in about a week, which are posted on your online dating profile.

Membership prices are 9 for three months, 9 for six months or 9 for 12 months. The company is testing two "markers" — the serotonin uptake transporter, involved in how people react to positive and negative emotions, and genes influencing your immune system.

Research shows there is a strong correlation between people in long-term relationships having different versions of the serotonin genes and different immune systems, said Ron Gonzalez, co-founder of Instant Chemistry."With online dating, you have socioeconomic factors people try to match on — religion, how much you make.

This is another layer on top of that so you can better find matches," Gonzalez said.

Studies have indicated that people with different MHC genes will actually smell better to you than people whose MHC genes are similar to your own.