What it is
A hair follicle is a tiny pouch in the skin from which a hair grows. In this condition lymphocytes gather in the follicle wall and damage it, so the hair no longer grows as it should and falls out. The affected skin builds up scale and can thicken or crust.
It is less a disease of its own than a way the skin reacts: it can appear by itself or accompany another illness hidden deeper in the body. That is why a vet looks for what set it off.
A cat may feel itchy, though not always. The hair loss itself does not hurt, but if your cat also eats less, loses weight or turns listless, it is a sign that the problem may lie beyond the skin.
Signs you may notice
The coat thins or falls out in patches, especially on the head, neck and around the eyes, and the skin flakes and may thicken. Sometimes the cat also becomes less lively and eats less.
When to see a vet
Book a visit if your cat is losing hair, or its skin is flaking or thickening, especially if it also turns listless or eats less.
This is general information, not veterinary advice. For your own cat's health, tests and treatment, see a vet.