There are many theories of why we have hair on our armpits. The first is that the hair deflects sweat away from the lymph nodes in the armpit and protects it from overheating. The second theory suggests that the hair lessens friction between the upper arm and thorax. The third theory is about sweating, and releasing sex pheromones.

The smell also known as B.O. is due to the by-products of microbial fermentation in your body’s damp creases. We all have bacteria living within our armpits that feed off the nutrient rich secretions that our bodies produce, creating waste that brings upon the nasty odor.

Researchers have found that our sweat suggests status within a group. It suggests gender, reproductive state in mammals, as well as sexual attraction. For example, if you are at a party and someone attractive comes near you, their smell can be the deciding factor to if you are still attracted to them or not.

The MHC in our body are a group of genes that are linked to our smell and immune system. In a study, a researcher asked a group of men to wear the same t shirt for a few days and later gave the tee shirts to another group of women and asked the women to choose a partner based upon the odor or their smelly t shirts. These women found that they were most attracted to the t shirts of men who had very different MHC’s to them, showing a great variation of genes, which is good for the immunity of the offspring that they could possibly create.

http://www.todayifoundout.com/index.php/2014/06/still-pubic-armpit-hair/

http://biology.stackexchange.com/questions/5217/what-is-the-armpit-hair-for