• alopecia areata (AA): common; [clinical patches of alopecia] (inflammatory...tending to orient at hair bulb).
• dermatophyte (tinea capitis) alopecia:
• androgenetic alopecia (AGA): common; [diffuse male pattern alopecia] (non-inflammatory). Increased proportion of hairs atrophy (miniaturized anagen follicles) and express toward short vellus-like hairs. On biopsy, hair follicles short & more superficial in dermis than normal.
• traction alopecia: common; [trichotillomania; hair abuse; hot comb alopecia: clinical patches to almost diffuse] (non-inflammatory...maybe the slightest follicular scarring; might see bent hair structures/follicles &/or streaks of follicular tract pigment in longitudinal histology).
• telogen effluvium: common; there are both classical acute TE (ATE) & chronic TE (CTE²) cases. Idiopathic ATE appears to have no inciting event. Postpartum ATE is an entity. The big DDX with CTE is vs. AGA. CTE is diffuse generalized thinning of abrupt onset but remains cyclically or steadily chronic.
  • hormonal deprivation or nutritional deficit alopecias: (non-inflammatory)...reduced anagen component by prematurely entering a resting, telogen phase. Sort of a CTE. Nutritional deficits usually include protein lack but may be due to lack of iron (iron deficiency).
  • stress alopecia: (non-inflammatory); stress causes growing anagen hairs to convert prematurely into resting telogen hairs, which means that more than the normal number of hairs are in the telogen phase and ready to shed & easily combed out. Akin to idiopathic acute telogen effluvium (ATE).
• toxic alopecia: (non-inflammatory); affects the anagen phase & due to chemo and some other toxins & even vitamin A overdose. Follicular production of the anagen stage hair shaft peters out within the follicle & hair "breaks" loose...shedding as if broken hairs.
• alopecia neoplastica: metastatic (lobular breast cancer) carcinoma cusing an AA-like alopecia.
• seborrheic dermatitis: (inflammatory); usually obvious as to cause, clinically; wouldn't expect a biopsy of this.

Scarring (cicatricial) alopecias¹:

• folliculitis decalvans: also known as alopecia folliculitis or acne decalvans, is an inflammatory reaction in hair follicles on the scalp that causes hair loss by micro-abscessing the air structure out. Akin to groin & axillary hidradenitis suppurativa. It leads to scarring and permanent hair loss. Maybe akin are "acne keloidalis" and "dissecting cellulitis of the scalp".
• tinea capitis: ring worm dermatophytic kerions (crusts of broken fungus, hairs, & ooze) can heal and leave alopecia patches due to follicular scarring.
• lichen planopilaris (LPP):  (usually inflammatory [interface lichenoid]...superficially folliculocentric). [S-05-9178?].Clinical correlation¹: various patterns of hair loss are seen, but most commonly there are scattered foci of partial hair loss. Perifollicular erythema and scaling are common to all cases. Lichen planus lesions elsewhere on the body support the diagnosis of LPP on the scalp. "Frontal fibrosing alopecia" appears to be just one of several clinical patterns of hair loss that can be seen in lichen planopilaris. However, lesions of lichen planus are not usually found in these patients, and the lichenoid inflammation does not affect the interfollicular epidermis.
• chronic cutaneous lupus (CCLE): [clinical patches of alopecia] (usually inflammatory...folliculitis/perifolliculitis, vacuolar basal epidermal change with underlying melanophages, thickened basement membranes).
• central, centrifugal scarring or cicatricial alopecia (CCSA or CCCA):clinically is an adult, most often a black woman, with a progressive, permanent loss of scalp hair starting on the central crown or vertex.... synonyms = FDS, folliculitis decalvans, tufted folliculitis, pseudopelade, hot comb alopecia.
• Brocq's alopecia (pseudopelade of Brocq): end stage scarring alopecia from many starting-point etiologies...too far gone (too close to end stage) for good clues as to etiology but maybe not so far as " burnt out scarring alopecia " (decreased total number of hairs, especially terminal hairs; loss of the sebaceous glands; residual, naked hair shafts surrounded by mild, granulomatous inflammation; follicular stelae without overlying follicles; and cylindrical columns of connective tissue at the sites of former follicles).
• keratosis follicularis spinulosa decalvans: