Apocrine glands are located in the skin, breasts, eyelids, and ears. They are a type of exocrine gland, which are glands that secrete hormones into a duct. Counterparts of exocrine glands are endocrine glands that secrete their hormones directly into the bloodstream. Certain endocrine glands also release hormones called pancrines that affect only specifically targeted cells very close to where they are released. Examples of endocrine glands are the pituitary gland, pancreas, adrenal gland, ovaries, and testes.
The modified apocrine glands located in the female breast are the milk-producing mammary glands. Additional, specialized glands located in the areola of the breast are responsible for secreting fat droplets into breast milk. Modified glands are located in the ears and aid in the formation of cerumen, more commonly known as earwax. In the skin and eyelids, the apocrine glands are sweat glands. The highest concentrations of apocrine sweat glands are located in the armpits and groin, and in the areola -- the darker skin surrounding the nipples of the breast -- of both human males and females.
While the bulk of actual sweat is produced by simple sweat glands called eccrine glands, apocrine glands in the skin act primarily as scent glands. These glands are considerably larger, are more deeply embedded in the skin, and produce a much thicker secretion than eccrine glands. Rather than cooling the body, the primary function of apocrine sweat glands is to produce a sexual scent that is as individual as a fingerprint. These glands become active once puberty sets in and play a vital part in sexual attraction.
The sex hormone scent, called a pheromone, enables communication with other members of the species by way of the olfactory sense, or sense of smell. One of the results of this silent, scent-based communication is sexual arousal. Even if humans aren't aware of it happening, it's been scientifically proven again and again that these pheromones do indeed influence our mating habits.
The scent produced by apocrine glands should not be confused with the unpleasant body odor produced by microorganisms that grow on moist sections of the skin. These microorganisms create body odor by digesting sebum, the oily substance secreted by sebaceous glands in the skin of mammals. The presence of water, in the form of sweat from eccrine (simple) sweat glands, aids in this process. Eccrine glands are activated by heat, explaining why we sweat more profusely as ambient temperatures rise. Apocrine glands react to stress and sexual activity, and respond by producing sweat with a personally characteristic -- but not unpleasant -- odor.