Cellulitis is formally an infectious process in the deep layers of the skin. A virus or, more commonly, a bacterium such as Staphylococcus aureus can get into the skin because of irritation caused by an allergic reaction. An ear is fairly resistant to such irritations, but every time you put an object or substance in, on, or through your ear, you risk breaking down that resistance. Only in the most severe cases is an auricular infection dangerous, however.
The external part of your ear has three structural areas, and the ones that are inflamed can help point doctors to the kind of process going on. The skin in the opening of the ear canal depends on a delicate balance of protective secretions and gentle cleaning to stay healthy. The cartilage of the upper ear has thin skin and little blood, but it can become infected. Cellulitis is most suspected when the fleshy lobe, which has no cartilage or bone and minimal secretions, is inflamed along with the skin in other areas of the ear. Any of these can start from an insect bite, scratch or other allergic provocation.
Family-practice physicians have faced rising rates of infections and allergic contact dermatitis as body piercings proliferate on the ear. As early as 1998, as many as 35 percent of people with pierced ears suffered infections, allergic reactions to their earrings or other complications. The lack of blood supply in the cartilage can make "high" piercings heal slowly. Nickel is the most commonly allergenic metal in earrings, though some people are sensitive to even gold and silver. In any case, broken skin is open to infection, and infected cartilage may not be repairable.
The ear canal slopes slightly down to drain moisture and the waxy cerumen secreted by some of its skin cells to protect against moisture. If the canal gets blocked, which can stem from an allergic reaction to chemicals in pool water or shampoo, or excessive cleaning with a scratchy object, materials that harbor infection can build up in the ear canal. When the material does drain, it can further irritate the skin outside the ear, or infection can travel out under the surface of the skin. The worst cases may call for systemic treatment, such as with oral antibiotics.
Dermatitis is inflammation of the skin from the outside in. It can be caused by an allergic reaction to plants, soaps or the plastic of a hearing aid. Eczema, formally atopic dermatitis, is a severe breakdown of the outer layers of the skin, and is thought to be caused by a combination of genetic, immunological and environmental factors. It is not common on the ear, but can spread close enough that the cracked and weeping skin it causes allows staph and other bacteria and viruses to enter the inner layers of skin, where infection can become systemic. Any other irritation established in the skin of the ear helps to open that door.