A pustule is a moderate stage of acne that is characterized by slight, localized swelling and visible pus. Pustules form in hair follicles when the pore becomes clogged and a tear forms in the follicular wall. Bacteria in the follicle trigger an immune response. White blood cells gather to destroy the bacteria. Inflammation, triggered by their presence, causes more white blood cells to arrive. The resultant accumulation of sebum, the oil that lubricates skin and hair, bacteria, white blood cells and other debris, is called pus.
Hormones
Certain normal body functions cause the body to increase the production of a class of hormone called androgens. Testosterone is an androgen that is often considered a "male" hormone, though women also make testosterone, albeit in lesser quantities. The American Academy of Dermatology (AAD) notes that androgens are responsible for most hormonally induced acne. During puberty in both boys and girls, menstruation, pregnancy and times of stress, testosterone levels in the body often increase. One effect of this increase is an increase in the production of sebum. With more sebum, comes the likelihood of more clogged follicles. Certain medications, such as birth control pills and steroids, also alter normal hormone production and so can lead to more clogged pores, and eventually, more pustules.
Bacteria
A number of bacteria normally live peacefully on the skin. In some cases, these bacteria trigger the immune response that causes pustules to form. Propionibacterium acnes (P. acnes) is one such bacteria that lives in sebaceous follicles. "Pub Med," a publication distributed by the U.S. National Library of Medicine, notes that a study of the P. acnes genome identified a variety of gene factors that are involved in triggering the inflammation that is associated with pustule formation.
Staphylococcus aureus is another bacteria that has been identified in pustule formation. People who carry this bacteria experience frequent outbreaks of a specific skin condition that causes pustules to form. Folliculitis is an inflammation of the hair follicle triggered by staphylococcus aureus. Common areas for this specific outbreak, include the beard area, groin area, buttocks and thighs.
Beauty Products
Even though they are supposed to "beautify," many skin and body care products contain ingredients that have been shown to irritate skin or clog pores. Lanolins, for instance, can irritate sensitive skin. When skin becomes irritated, an inflammatory response is triggered and pustules can form. Other ingredients are occlusive. This means they form a barrier on the surface of the skin. There can be benefits to this in that barrier ingredients keep excessive moisture from evaporating from the skin. The down side is that some of these same ingredients block sebum from naturally escaping from the hair follicle. These blockages can eventually lead to pustule formation.
