You're probably very aware of any unfamiliar bumps on your scalp --- after all, you're performing a self-examination every time you shampoo your hair. If you feel a strange bump or growth on your scalp, it's a good idea to see your doctor. In most cases, the bumps will be harmless and treatable, as in the case of a benign cyst, but only your doctor can diagnose you with certainty.
According to London's Holborn Hair & Scalp Clinic, an allergic reaction can be the cause of your scalp bumps. Called contact dermatitis, these allergic reactions come in two types: irritant contact dermatitis, which occurs after a single exposure to an allergen, and allergic contact dermatitis, which occurs after repeated exposure. Contact dermatitis often results in swelling, blisters and a rash, triggered by such substances as hair dye or some hair-care product ingredients. The clinic notes that with allergic contact dermatitis, you'll usually notice symptoms about 12 hours after a seemingly routine exposure to the irritant.
Pilar cysts are harmless skin-colored bumps that appear most frequently on the scalp. According to the University of California at San Francisco's Dermatology Glossary, a pilar cyst forms in a dome-like shape, with smooth skin stretching over the bump. The bump is usually firm but not hardened --- it's filled with keratin, a protein. Cysts are harmless, but usually must be removed by a doctor.
According to the American Academy of Dermatology, seborrheic keratoses are small, wart-like bumps that usually crop up on the back, chest and scalp. These bumps are light brown, dark brown or black. They have a slightly shiny look to them, and although scientists haven't determined exactly what causes them, they seem to strike people whose family members also have them or people who have had a great deal of sun exposure.
In the case of an allergic reaction, the Merck Manuals Online Medical Library notes that washing your scalp with soap and water can help remove traces of the irritant. Until you identify what it is, however, you run the risk of reapplying it and continuing the allergic reaction. When in doubt, contact your doctor and discontinue use of any new hair products or new fabric items such as hats, scarves or sheets that could contain irritation-causing dyes. If the bump is a harmless growth such as a cyst or keratosis, your dermatologist likely will use cryotherapy to "burn" the bump away, or traditional surgical techniques to cut it off.
In rare cases, bumps on the scalp can be cancerous growths. According to the American Academy of Dermatology, seborrheic keratoses look similar to another form of skin lesion called actinic keratoses, as well as to some forms of melanoma. To be safe, have any bump on your scalp inspected by a dermatologist; should the bump be an early form of skin cancer, a quick diagnosis is the first step toward successful treatment.