A primary lesion is a change in the skin that is caused by a certain disease or infection. There are several different types, including the macule, papule, vesicle, plaque, bulla, patch, tumor, wheal, nodule, and pustule. These are not specific to one disease, and are not always considered to be a primary lesion. Changes that occur within these kinds of lesions are known as secondary lesions.
Macules are small areas of skin that have changed color. They cannot be felt, only seen. A patch is a primary lesion that is similar to the macule, but takes up a bigger area of the skin. Patches are generally larger than one cm, while a macule is typically smaller than one cm.
A papule, a bulla, and a vesicle are all small, raised lesions on top of the skin. Papules are solid, have shape, and are often accompanied by secondary lesion qualities, such as scaling and crusting. This type of lesion also has a raised border around it. Vesicles and bullae are not solid and are filled with a clear fluid.
Tumors are generally bigger than two cm. These lesions are solid and made of subcutaneous tissue or skin. They may also be malignant. A nodule is similar to a tumor, but smaller. It can be composed of skin, subcutaneous tissue, or dermis.
Pustules are elevated lesions that have pus within them. Many times they contain infection, but in some cases do not. They are small and generally do not grow bigger than one cm.
A plaque is a raised, solid lesion that is typically bigger than one centimeter around. Wheals are sometimes similar to plaques, but may also be similar to papules. A wheal has a border around it and often has a center that is very pale.
Another kind of primary lesion is called a burrow. This refers to tunnels under the skin, typically due to an infestation of some type of parasite. These tunnels appear as straight lesions on the top of the skin. It is most commonly seen in cases of scabies.
A primary lesion is generally not associated with one specific disease. Many times, different diseases will produce the same type of basic lesion. Identifying the primary lesion and its cause is usually the first step that a doctor takes in treating or curing a disease.