Lichen planus is an itchy but harmless rash that usually affects adults. The name comes from the appearance of the rash, which forms as small, flattish bumps. Lichen refers to the bumpiness, while planus simply means flat.

An Autoimmune Response

We don’t understand the causes of lichen planus completely, but there are a few clues that suggest it could be linked to the immune system. When your immune system detects something it sees as a threat, it starts producing antibodies. The antibodies can attack viruses or bacteria to eliminate an infection. However, sometimes the immune system creates lots of antibodies that attack your own cells and cause inflammation instead. This is known as an autoimmune response, and it seems to be responsible for lichen planus.

A Bad Reaction

One reason why we believe lichen planus to be linked to an immune system reaction is that the same kind of rash can sometimes appear when people take certain kinds of medication. Some people develop a lichen planus rash when they are taking antimalarial pills or undergoing certain treatments for high blood pressure or arthritis. This tells us that an allergic or immune response can produce the rash in some people, so it could also be responsible for the other cases.

What We Know So Far

While there is still a lot that we don’t know about lichen planus, the good news is that it isn’t genetic and you can’t catch it. While a tendency to develop lichen planus does seem to run in some families, there is no guarantee that you’ll get it just because someone you are related to has had it. There is also no risk of catching it from another person, even when you are in close contact. We might not know exactly what causes lichen planus, but we are certain that it is not infectious.