Pheromones

I got this from 'How Stuff Works'. Its a very interesting read on Pheromones and its connection with the immune system.

The debated topic of human pheromones still carries some weight in the field of love research. The word "pheromone" comes from the Greek words pherein and hormone, meaning "excitement carrier".

In the animal world, pheromones are individual scent "prints" found in urine or sweat that dictate sexual behavior and attract the opposite sex. They help animals identify each other and choose a mate with an immune system different enough from their own to ensure healthy offspring. They have a special organ in their noses called the vomeronasal organ (VNO) that detects this odorless chemical.

The existence of human pheromones was discovered in 1986 by scientists at the Chemical Senses Center in Philadelphia and its counterpart in France. They found these chemicals in human sweat. A human VNO has also been found in some, but not all, people. Even if the VNO isn't present in all of us -- and may not be working in those who do have it -- there is still evidence that smell is an important aspect of love (note the booming perfume industry). An experiment was conducted where a group of females smelled the unwashed tee shirts of a group of sweaty males, and each had to select the one to whom she was most "attracted." Just like in the animal world, the majority of the females chose a shirt from the male whose immune system was the most different from their own.