The skin represents about 15% of body mass and is the largest and most visible organ in the human body. However, many functions of the skin are often overlooked, including its role as a protective barrier against the sun, a shield against germs, a reservoir for vitamin D, and a means of regulating body temperature closely. The skin also provides a view of the underlying body tissues it protects. Disorders of the intestines, blood, hormones, and even the heart can first manifest on the skin in the form of a rash.
**Erythema Migrans**
Ticks are annoying creatures, and while the vast majority of tick bites won't make you sick, there is one rash that should prompt you to visit your doctor if you notice it. Erythema migrans is a rash notable for its ability to expand rapidly across the skin and is the hallmark of the potentially severe bacterial disease known as Lyme disease. This rash has a classic target pattern, resembling the bullseye on a dartboard. Be vigilant for a few weeks after a tick bite to ensure that this rash does not appear, especially if you notice a red bump that wasn't there before or if you had to remove a tick from your skin. You should also monitor for other symptoms associated with Lyme disease, such as fluctuating fevers, muscle and joint pains, and headaches.
**Purpura**
The name of this rash is derived from the mollusks that were used to create purple dye. Purpura refers to a rash of small purple or red spots. The cause lies in blood pooling in the deeper layers of the skin (the dermis). Purpura indicates a problem either with the walls of the small blood vessels that nourish the skin or with the blood contained within them. This may be due to a deficiency in platelets, the small cell fragments that allow blood to clot, potentially caused by bone marrow failure or an autoimmune condition where the body attacks its own cells. In the worst cases, purpura may indicate a life-threatening condition of sepsis, where an infection spreads to the bloodstream, possibly from the lungs, kidneys, or even from the skin itself.
**Spider Nevi**
Spider nevi represent an issue within the skin's small blood vessels (the small arteries that supply blood to the skin). These arteries open and close to regulate heat loss from the body surface. However, sometimes a spider-like pattern can appear, where the open artery represents the body of the spider, and the smaller, radiating capillaries are the spider's legs. Often, these growths are benign and not associated with any specific condition. However, more than three spider nevi may indicate elevated estrogen levels in the bloodstream, often due to liver diseases or hormonal changes occurring during pregnancy.
**Acanthosis Nigricans**
Changes in skin folds (usually around the armpits or neck)—especially if they become thick and velvety to the touch—may indicate a condition known as acanthosis nigricans. This "black velvety" appearance of the skin is more commonly seen in darker skin types. Typically, the condition is linked to metabolic disorders such as type 2 diabetes and polycystic ovary syndrome. If either of these conditions is successfully treated, the rash may fade. In rare cases, it can also be a sign of stomach cancer.
