The injection of sebum and comedones into the human skin excites a lymphocytic inflammatory response. The free fatty acids in sebum are chiefly responsible for the observed inflammation. When the inflammation is quite marked, the pilosebaceous follicles undergo rupture, and strands of epithelium grow out to encapsulate the inflammatory mass. Giant cells, which may be present in the inflammatory response, appear to be the result of follicular collapse with the consequent liberation of follicular wall keratin and hair which act as foreign bodies. These histological changes are comparable to those occurring in acne.