A canker sore is an ulceration of the tissue in the mouth or throat. The ulcer contains fibrin and inflammatory white blood cells.
A canker sore is another term for an oral apthous ulcer. There is a disruption of the mucous membrane of the mouth or throat that is often painful. They are caused by citrus fruits, trauma, weight loss, food allergies, medications and the immune system. Celiac disease, Behcet's disease, HIV and Crohn's disease are some conditions associated with canker sores.
Apthous Ulcer Structure
An apthous ulcer often begins as a bump that then becomes an open sore. There is an erythematous ring (red) that develops around a white or yellow oval. The area of redness is fibrin, a protein the body makes that's involved with clotting. It's part of the body's response to heal the ulcer. Numerous inflammatory white blood cells are present in and around the ulcer.
Minor ulcers are between 3 mm (0.1 inches) and 10 mm (0.4 inches) in size. They have a ring of redness surrounding a yellow-gray center.
Major ulcers are greater than 10 mm in size. They have a similar appearance to minor ulcers.
These ulcers occur as numerous 1 to 3 mm lesions that develop in clusters.
Canker Sore vs. Cold Sore
A canker sore is not due to herpes simplex. Herpes simplex creates cold sores. Cold sores contain the herpes simplex virus, while apthous ulcers do not contain any virus.