The structure consisting of hair, hair follicle, arrector pili muscles, and sebaceous gland is an epidermal invagination known as a "pilosebaceous unit".
If eyelashes are not kept clean, conditions such as folliculitis may take place, and if the sebaceous gland becomes infected, it can lead to abscesses and styes.
A water impermeable secretion produced by a sebaceous gland covers the feathers and the birds spread it across their body using their beak or their head.
