A canker sore is a shallow wound in the soft tissue of your mouth, made up of a layer of dead cells and protein debris (called fibrin) on the surface, surrounded by a ring of inflamed, blood-rich tissue underneath. That white or yellowish center you see isn’t pus or infection. It’s a mix of fibrin, dead mucosal cells, and immune cells that have flooded the area, all sitting in a small crater where the top layer of tissue has broken down.
What Creates the White Center
The surface of a canker sore is covered by a thin layer of exudate, a film made mostly of fibrin (the same protein your body uses to form blood clots) mixed with cellular debris. This coating is what gives the sore its distinctive white or yellowish appearance. Beneath that film, the tissue is actively being broken down by your own immune system.
In the early stages, the area fills with a type of white blood cell called mononuclear cells, which are part of your body’s targeted immune response. These cells attack the lining of the mouth as though it were a threat. As the ulcer matures and bacteria from the mouth colonize the open wound, a second wave of immune cells (neutrophils) moves in. This secondary bacterial involvement can make the sore more painful and slightly swollen, but it’s a consequence of the ulcer, not the cause.
Why Your Immune System Attacks Healthy Tissue
Unlike cold sores, which are caused by the herpes simplex virus and contain viral particles inside fluid-filled blisters, canker sores have no known infectious cause. They contain no virus, no bacteria driving the initial damage, and no contagious material. The destruction is entirely self-inflicted by your immune system.
The leading theory is that certain immune cells misidentify the thin mucosal lining inside your mouth as something harmful. These cells release inflammatory chemicals that break down the surface tissue, creating the open crater. What triggers this misfiring varies from person to person, but common catalysts include minor trauma (biting your cheek, a sharp chip, aggressive brushing), hormonal shifts, stress, and nutritional gaps. Deficiencies in iron, folate, and vitamin B12 are all linked to recurring mouth ulcers, likely because these nutrients play key roles in maintaining healthy mucosal tissue and regulating immune function.
The Red Ring Around the Edge
The inflamed border surrounding the white center is hyperemic tissue, meaning it’s engorged with extra blood flow. Your body sends blood to the area as part of the inflammatory response, delivering more immune cells and nutrients for repair. This red halo is what makes the sore so sensitive. The nerve endings in that zone are exposed to the inflammatory chemicals flooding the tissue, which is why even mild contact with food, toothpaste, or acidic drinks can cause sharp, disproportionate pain.
How Canker Sores Differ From Cold Sores
Because people often confuse the two, it helps to understand that these sores are made of entirely different material. Cold sores are fluid-filled blisters caused by herpes simplex virus (usually HSV-1). They form on the outside of the mouth, around the lips, and contain viral particles that make them highly contagious. Canker sores are dry, open ulcers that form only inside the mouth, on the inner cheeks, lips, tongue, or soft palate. They have no viral component and are not contagious at all. You cannot spread a canker sore to another person through kissing, sharing utensils, or any other contact.
Three Types, Three Different Sizes
Not all canker sores are built the same. They come in three forms, and the difference is mostly about scale.
- Minor aphthous ulcers are the most common type. They measure less than 5 millimeters across (roughly the size of a pencil eraser) and heal on their own within 10 to 14 days without scarring.
- Major aphthous ulcers exceed 1 centimeter in diameter and penetrate deeper into the tissue. These can take up to 6 weeks to heal and sometimes leave scars because the damage extends below the surface layer of the mucosa.
- Herpetiform ulcers start as clusters of tiny sores, sometimes up to 100 at once, that merge together into large, irregularly shaped wounds. Despite the name, they have nothing to do with herpes virus.
The composition of all three types is the same: fibrin-coated surface, immune cell infiltration, and inflamed surrounding tissue. The difference is how deep and how wide the immune attack goes.
What Helps Them Heal
Because canker sores are driven by inflammation rather than infection, treatment focuses on calming the immune response and protecting the exposed tissue. Prescription dental pastes containing a mild steroid can reduce the inflammatory activity, easing pain and potentially shortening healing time. These are typically applied after meals and at bedtime, when the paste can stay in contact with the sore without being washed away by food or drink.
Over-the-counter options work by creating a protective barrier over the ulcer’s surface, shielding those exposed nerve endings from irritation. Rinsing with warm salt water can also help by gently reducing bacterial colonization on the open wound, which limits the secondary inflammation that makes sores worse in their later stages.
For people who get canker sores frequently, addressing underlying nutritional deficiencies can reduce how often they appear. If you notice a pattern of recurring ulcers alongside fatigue or a persistently sore tongue, testing for iron, folate, and B12 levels is a practical starting point. Canker sores affect anywhere from 5% to over 40% of the population depending on the group studied, so recurrent episodes are common, but they’re also one of the more treatable patterns once the underlying trigger is identified.