Did you know that your skin is the largest organ of your body? It is, in terms of both weight—between 6 and 9 pounds—and surface area—about 2 square yards. Your skin separates the inside of your body from the outside world. It protects you from bacteria and viruses, and regulates your body temperature.

Conditions that irritate, clog, or inflame your skin can cause symptoms such as redness, swelling, burning, and itching. Allergies, irritants, your genetic makeup, and certain diseases and immune system problems can cause dermatitis, hives, and other skin conditions. Many skin problems, such as acne, also affect your appearance. Your skin can also develop several kinds of cancers.

Here are the key facts about some of the most common skin problems:

Acne—A disease that affects the skin’s oil glands. The small holes in your skin (pores) connect to oil glands under the skin. These glands make a substance called sebum. The pores connect to the glands by a canal called a follicle. When the follicle of a skin gland clogs up, a pimple grows. Acne is the most common skin disease; an estimated 80 percent of all people have acne at some point. Early treatment is the best way to prevent scars. Your doctor may suggest over-the-counter (OTC) or prescription drugs.

Eczema—Also known as atopic dermatitis, this is a long-term skin disease. The most common symptoms are dry and itchy skin, rashes on the face, inside the elbows, behind the knees, and on the hands and feet. Currently, there is no single test to diagnose eczema, so doctors rely on information about you and your family.

Hives—Red and sometimes itchy bumps on your skin. An allergic reaction to a drug or food usually causes them. People who have other allergies are more likely to get hives than other people. Other causes include infections and stress. Hives are very common. They usually go away on their own, but if you have a serious case, you might need medical help.

Melanoma—A severe and potentially life-threatening skin cancer. The “ABCD’s” of what to watch for with the moles on your skin:

  • Asymmetry: the shape of one half does not match the other
  • Border: the edges are ragged, blurred, or irregular
  • Color: the color is uneven and may include shades of black, brown, and tan
  • Diameter: there is a change in size, usually an increase