A callus is an overgrowth of keratin, the same protein as in fingernails. Constant rubbing or pressure stimulates the growths to protect the underlying skin from further irritation. A callus is typically less sensitive to touch than the normal skin surrounding it but can be painful if it becomes infected.
There are two major types of calluses: discrete nucleated and diffuse-shearing. The first is a localized lesion with a central keratin plug. It is typically sensitive to touch and may be mistaken for a wart. Diffuse-shearing calluses usually measure over 1 centimeter across with no keratin plug and are usually painless.
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