Infections inside the skin or on its surface or an allergic reaction cause skin lesions, Healthline explains. In addition, skin lesions may be hereditary or already present at the time of birth.
Skin lesions may be primary or secondary, Healthline notes. Primary skin lesions are already displayed at birth or appear later in life on healthy skin, while secondary lesions are irritated or scratched primary lesions. Primary skin lesions include birthmarks, macules, blisters, nodules and rashes. Macules, or flat moles or freckles, are small circular spots that are white, red or brown. Blisters are tiny pouches with clear fluid that may develop due to sun damage, a viral infection, friction with fabric, or an insect bite. A nodule is a raised, relatively large lesion with a diameter of over 2 centimeters. A rash extends over an area of skin that may be quite large, and it is typically the result of an allergic reaction.
Secondary skin lesions include scars, scales, ulcers and crusts, Healthline states. Scars that do not fully heal into normal skin again are called keloids. Scales are buildups of skin cells that eventually fall from the surrounding area. An ulcer forms either due to physical damage or bacterial infection. A crust develops when a primary lesion is scratched or rubbed, causing dried blood to cover it.