Everything Else . The most likely culprits for yellow clothing stains are apocrine sweat and a body product that often shows up in places that also have apocrine sweat glands, the oily secretion of the sebaceous glands, called sebum. 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. The underarms and groin are rich in apocrine sweat glands, which produces secretions that are milky and usually odorless until acted on by bacteria. Appocrine 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.