The most likely culprits for yellow clothing stains are body secretions called apocrine sweat and sebum, the oily secretion of the sebaceous glands.
Deodorants and antiperspirants may also play a role.
The sebaceous glands are usually associated with hair follicles. Cells filled with fatty droplets die and burst, providing lubrication for the skin and hair.
When the oils are exposed to air, they oxidize, turning yellowish, and if not quickly removed by thorough laundering, they can permanently yellow clothing. Sebaceous glands at the back of the neck cause “ring around the collar.”
The underarms and groin are rich in apocrine sweat glands, which produce secretions that are milky and usually odorless until acted on by bacteria. Apocrine sweat contains many chemicals, including the acidic substances that produce the characteristic underarm odor.
The most copious human sweat, the kind produced by widely distributed sweat glands called eccrine glands, is probably not a big stain producer unless the skin or clothing is already soiled. This kind of sweat is the body’s chief means of temperature regulation, providing for cooling by evaporation.
It is about 99 percent water, with only about 1 percent of it consisting of salt, urea, ammonia, and uric acid.