Alopecia is an abnormal hair loss that leads to thinning or disappearance of hair in certain areas of the head or trunk. Types of alopecia are classified depending on the course of the pathological process (presence or absence of scarring) and on the degree of its prevalence (limited or cicatricial alopecia).
Cicatricial types of alopecia are the result of the active destruction of the hair follicle and its replacement with fibrous tissue. As a rule, scalp diseases occur in two phases: in the early stages, non-scarring alopecia develops – a type of hair loss, accompanied by complete loss of the bulbs and their subsequent replacement with fibrous tissue. Then the cicatricial form appears. It, in turn, is divided into primary, in which the follicle itself becomes the target for the inflammatory process, and secondary when the damage to the hair follicle is a consequence of nonspecific inflammation. Primary alopecia types include such inflammatory diseases of the scalp area as lupus erythematosus, lichen planus, keloid acne, which are based on overactive reactions of cellular immunity. Secondary cicatricial alopecia types are such types of hair loss that develop against the background of trauma, burns, tumors, and radiation therapy. In both cases, the areas of baldness are smooth scar, devoid of follicles, therefore, restoration of hair growth in such areas is impossible.
Non-scarring types of baldness develop as a result of a disruption in the hair growth cycle. They are most often genetically predetermined, less often they can be caused by external causes (for example, pharmaceutical drugs). Telogen types of hair loss are associated with a premature onset of the resting phase and an increase in the number of the falling out telogen hair that has lost the ability to grow. This group includes genetically predetermined hair loss, provoked by the action of androgens circulating in the blood and leading to a programmed reduction and death of the bulbs. With anagen types of alopecia, hair loss occurs due to follicles in the phase of active growth. Generally, these are severe types of alopecia, in some cases leading to complete hair loss. The cause may have autoimmune nature when immune cells do not distinguish between their own proteins and have an aggressive effect on growing shafts. A striking example of this type of baldness is alopecia areata. At the same time, anagen alopecia can develop against the background of the toxic effect of pharmaceutical drugs used, in particular, in chemotherapy of tumors that disrupt the division and metabolism of hair cells.