Such luminaries as Hippocrites and Aristotle noticed that eunuchs don't go bald. Excessive DHT causes an inflammation that damages hair follicles. It is also a major component of Male 40 which contains many other ingredients beneficial for this condition. Atherosclerosis, by inhibiting blood flow, also contributes to baldness, and people with this disorder have a higher incidence of baldness. See the page on Cardiovascular disease for solutions to this problem.
Antibodies exist to hair follicles and hair that has fallen out resembles a rejected organ.
To be sure that the immune system has not malfunctioned see the page on immune dysfunction for answers. Anecdotally, the immune modulator, Moducare , is growing hair on bald heads. Those who may despair that they will ever have hair, have effective remedies now available. Though not as publicized as Minoxidil they are far more effective. Dr. Peter Proctor has formulated some products which are far more effective than minoxidil and are so advanced that he has patented them before the big drug companies could do so.