Alopecia areata (patchy baldness) is a noncontagious, autoimmune disease in which white blood cells release chemical messengers known as cytokines to attack healthy cells located in hair follicles. This shrinks the follicle to a point where hair production is considerably reduced. However, new follicle cells being produced naturally by the body are not affected, making hair regrowth possible in all cases.

Exactly why alopecia areata occurs is unknown. Some research suggests a combination of certain genes may make some people more likely to develop the disease. It may be that some sort of biological (e.g., virus) or emotional trigger (e.g., stress) initiates alopecia areata in people genetically predisposed to the disease. People with family members affected by the disease have an increased risk for exposure, particularly if the initial hair loss occurs before the age of 30.

Children with the disease do not usually have a parent with the condition. It is generally believed that several genes cause alopecia areata. This makes it unlikely that a child born to one or both parents with alopecia areata would inherit all of the necessary genes to develop the disease. In addition, twins with identical genes that cause alopecia areata often do not develop the disease in unison, suggesting other factors besides genetics may play a role in how the disease is triggered.

People with asthma, thyroid disease or certain allergic conditions (e.g., atopic eczema, hay fever) are also at increased risk for alopecia areata. The condition also occurs more frequently in people with a family history of these conditions or other autoimmune diseases (e.g., diabetes, rheumatoid arthritis, vitiligo). However, patients with alopecia areata do not usually have other autoimmune diseases themselves. The condition is also common in people with Down syndrome or other chromosomal disorders.