The Difference between Psoriasis and Eczema

Psoriasis and eczema are two common skin conditions that tend to run in families. They are characterized by red skin, scaling, and itching, and usually relapse over time. Both psoriasis and eczema generally respond to topical steroid medications. These two conditions are often confused, yet have different characteristics.

Psoriasis usually develops in adult life, whereas 90% of eczema cases develop before the age of 5 years. Psoriasis affects three percent of the population overall, and eczema occurs in 10 to 20% of children, but remains a problem for only one percent of adults.

Importantly, psoriasis and eczema result from two different immune processes. Psoriasis is an autoimmune disease thought to be mediated by Th1 and Th17 immune cells, leading to chronic skin inflammation. Psoriatic lesions can be triggered by a variety of environmental factors such as strep throat, injury to the skin, or even smoking. Psoriasis primarily shows skin lesions, but it can also manifest as joint disease.

Compared to psoriasis, eczema is thought to be mediated by Th2 cells, a different subset of immune cells, which are responsible for allergic responses to environmental allergens. People with eczema have a higher chance of having asthma and/or hay fever, constituting what is often called the “atopic triad.” Children with eczema are often sensitive to cow’s milk, eggs, peanuts and/or soy. As adults, common allergies for those with eczema include dust mites, pollen, animal fur, and mold. It should be stressed, however, that these allergic reactions are not necessarily related to eczema, and that skin or blood tests for allergens are not generally helpful in finding an underlying trigger for most people.

Eczema skin lesions frequently show Staphylococcus aureus, a common bacterium, even if there is no evidence of infection. When this bacteria overgrows, scratched areas of eczema often become infected. Widespread viral infections can also occur with eczema. These include herpes simplex virus infections, also known as cold sores, and molluscum, a wart-like viral infection characterized by smooth, dome-shaped bumps.

In psoriasis, skin cells are produced at an exceptionally rapid rate, causing an abnormal amount of cells in the top layer of the skin. This results in thickened, flaky, silver-colored lesions that are typically seen in plaque psoriasis, the most common type of psoriasis.

Eczema skin, in contrast, is characterized by severe dryness and an impaired function of the top skin layer, which results in significant water loss. Eczema skin becomes red, flaky, and blistered. Many eczema sufferers feel the need to itch the skin, and this can result in scars and markings.

Both psoriasis and eczema may worsen during the winter, but for different reasons. While cold weather may worsen skin dryness and therefore allow eczema lesions to develop, decreased exposure to ultraviolet sunlight, which is helpful in calming the lesions of psoriasis, may exacerbate psoriatic symptoms.

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