What causes a callus to form on the skin?

A callus forms on the skin primarily as a response to excessive friction or pressure. When the skin is subjected to repeated stress or abrasion, it undergoes a thickening process called hyperkeratosis. This is the body’s natural defense mechanism to protect the underlying tissues from damage. The increased thickness of the outer layer of skin effectively creates a barrier against further irritation, making the area tougher but also potentially less sensitive.

In contrast, other factors listed, such as excessive moisture, infection, and allergies, do not typically lead to the formation of a callus. Excessive moisture can cause skin maceration or softening, while infections can lead to different skin conditions, and allergies generally cause inflammation or irritation rather than thickening. Thus, the presence of friction or pressure is clearly the primary cause of callus formation.