A: ‘Dermatitis’ vs. ‘Eczema’. Lets take these words one at a time.
‘Dermatitis’ is a non-specific term. It refers to many inflammatory skin conditions, but especially to those diseases that affect the outer compartment of skin, the ‘epidermis’. One very common type of dermatitis is atopic dermatitis, which affects ~20% of infants and children and often occurs in otherwise “allergic” individuals, who have or later on develop asthma and/or hay fever.
Another common type of dermatitis is contact dermatitis. Contact dermatitis is a reaction to a substance that comes in direct contact with the skin. It can be allergic in nature – for example, the rash from contact with poison ivy, poison oak or poison sumac, which occurs in people who have become allergic or ‘sensitized’ to the antigen, urushiol, found in the plants’ leaves and stems. Or it can be an irritant response, meaning that the chemical is harsh itself, and its reaction doesn’t rely on the immune system’s recognition that it is a foreign chemical. Hydrochloric acid would be an example of a chemical capable of provoking an irritant dermatitis. With exposure to a sufficient quantity of the acid, it would cause a rash on virtually anyone’s skin. Not all irritant skin reactions, though, are caused by such harsh chemicals.
Soaps are a good example of irritants that only bother some people — namely those with very sensitive skin.
There are several other types of dermatitis, as well: ‘seborrheic dermatitis’ (a common, recurrent condition typically found on body sites rich in oil (‘sebaceous’) glands), and ‘stasis dermatitis’ (typically found on the lower legs of individuals with venous insufficiency and chronic swelling (‘lymphedema’)), to name a couple of the more common types.
“Eczema” is also a non-specific term, although it is often used to refer specifically to “atopic dermatitis”. By definition, eczema denotes a form of skin inflammation in which fluid builds up between the cells of the epidermis. This usually can only by appreciated with the magnification of a microscope. One sees a pulling apart of epidermal cells, producing a picture pathologists term “spongiosis”. But the fluid accumulations in an eczematous dermatitis can be more pronounced and even produce blisters visible to the naked eye. Many of us may have experienced this with allergic reactions to poison ivy or poison oak.
To a large extent, all of the above examples of dermatitis are also eczemas. But psoriasis, another common inflammatory condition of the epidermis, is not eczematous. Fluid accumulation between epidermal cells (‘spongiosis’) is not a characteristic feature of psoriasis under the microscope. Hence, psoriasis is a kind of dermatitis, but not an eczema.
Given this nonspecific and confusing terminology, it is always preferable to use the most specific term that we can when referring to a skin disorder.