Inflammation of any part or all parts of the hair follicle is categorised as having this pattern. The variety of diseases are manifest clinically as alopecia, scaling skin disease, or nodular skin disease.
The pathogenesis of this group of diseases is similar in many respects. The target of the inflammation is either the epithelial cells of the follicle (sebaceous adenitis, mural folliculitis, bulbitis) or the lumen of the follicle (luminal folliculitis). The cause is either infectious or noninfectious.
This group of diseases is divided into 2 main areas based on anatomic location - folliculitis and furunculosis, and sebaceous adenitis. Furunculosis occurs when folliculitis is very severe, causes rupture of the follicle and the inflammation spills out into the surrounding dermis.
The target of inflammation of the follicle is either the wall or the epithelial cells, or the lumen of the follicle and infectious agents therein. Types are subdivided according to the target of the inflammation (luminal or mural), on the cell type (neutrophilic, eosinophilic, lymphocytic or granulomatous), or on matrix type (mucinous, lipid)
Luminal folliculitis is mostly infectious but can be a hypersensitivity reaction (especially eosinophilic folliculitis). The infectious microbe or agent is either a bacterium, dermatophyte or larger parasite, with Demodex being the most common.
Bacterial folliculitis
Dermatophytic folliculitis
Demodex
Sebaceous adenitis is an immune attack on the sebaceous gland. It is a very specifically targeted disease. It is reported in many different species,
Early lesions are lymphocytic, however spillage of sebum into tissue quickly attracts macrophages and multinucleated giant cells.
End stage lesions have complete destruction of the sebaceous glands, lack of the lipid barrier and sometime loss of the hair follicle stem cells and follicular atrophy and alopecia.