Burst infected sebaceous cyst. View of a large, inflamed lump on a patient's neck. This is an infected sebaceous cyst which has burst revealing its contents of pus and sebaceous material (yellow). A sebaceous cyst forms when the secretory gland associated with a hair follicle (sebaceous gland) becomes blocked. The gland enlarges as it fills with sebum (oily substance that lubricates and waterproofs skin) forming a nodule under the skin. Although harmless, sebaceous cysts may grow very large, or become infected and painful (as here). Treatment may involve removal of the cyst, with antibiotics (antibacterial drugs) to treat the infection. See M130/338 for appearance of cyst before bursting.