The quality of sebum generated by these glands changes and makes the environment more suitable for the growth of bacteria (Cutibacterium acnes).
In addition to excessive and abnormal sebum production, the dead cells of the superficial layer tend to clog the opening of the pilosebaceous unit.
The combination of these factors makes the glands swell, resulting in painful reddish spots on the face, back, chest, arms, buttocks, scalp etc. The environment inside the gland opening is determined by the genetics, bacterial load, regularity of exfoliation, blood glucose levels, hormones etc.