There are three types of sweat glands: eccrine, apocrine, and apoeccrine. Apocrine glands are primarily responsible for body malodor and, along with apoeccrine glands, are mostly expressed in the axillary (underarm) regions, whereas eccrine glands are distributed throughout virtually all of the rest of the skin in the body, although they are also particularly expressed in the axillary regions, and contribute to malodor to a relatively minor extent. Sebaceous glands, another type of secretory gland, are not sweat glands but instead secrete sebum (an oily substance), and may also contribute to body odor to some degree.
The main odorous compounds that contribute to axillary odor include:
# Unsaturated or hydroxylated branched fatty acids, with the key ones being (E)-3-methyl-2-hexenoic acid (3M2H) and 3-hydroxy-3-methylhexanoic acid (HMHA)
# Sulfanylalkanols, particularly 3-methyl-3-sulfanylhexan-1-ol (3M3SH)
# Odoriferous androstane steroids, namely the pheromones androstenone (5α-androst-16-en-3-one) and androstenol (5α-androst-16-en-3α-ol)
These malodorous compounds are formed from non-odoriferous precursors that are secreted from apocrine glands and converted by various enzymes expressed in skin surface bacteria. The specific skin surface bacteria responsible are mainly Staphylococcus and Corynebacterium species.
The androstane steroids dehydroepiandrosterone sulfate (DHEA-S) and androsterone sulfate have been detected in an extract of axillary hairs together with high concentrations of cholesterol. Apocrine sweat contains relatively high amounts of androgens, for instance dehydroepiandrosterone (DHEA), androsterone, and testosterone, and the androgen receptor (AR), the biological target of androgens, is strongly expressed in the secretory cells of apocrine glands. In addition, 5α-reductase type I, an enzyme which converts testosterone into the more potent androgen dihydrotestosterone (DHT), has been found to be highly expressed in the apocrine glands of adolescents, and DHT has been found to specifically contribute to malodor as well. Starting at puberty, males have higher levels of androgens than do females and produce comparatively more axillary malodor. As such, it has been proposed that the higher axillary malodor seen in males is due to greater relative stimulation of axillary apocrine sweat glands by androgens.