People infected with Lyme Disease often present with an erythema migrans ("migrating redness") rash. Most often, these rashes are in the shape of a bulls-eye.

Rash image.

Presumably, this is a reaction that many of our bodies have to the Borrelia burgdorferi pathogen.

Lyme disease also sometimes presents with different-looking rashes, and other pathogens also sometimes produce a bulls-eye-looking rash. CDC Link on Lyme Disease Rashes and Look-alikes.

What causes the bulls-eye shape of these rashes?

Thought process: A gradient of redness from the site of the tick bite makes sense--an immune response might be strongest at the bite site--but I'm still stumped on why there would be a spot of redness surrounded by a no-rash-ring surrounded by a rash-ring.

Note: If I remember correctly, external links to images are usually frowned upon. Although the images linked in this post aren't super graphic, I decided to make them external links for the sake of anyone who is sensitive to medical images.


The spreading of the rash is caused by dissemination (migration) of the Lyme spirochetes (Borrelia) through the skin out from the initial site of infection.

The spirochetes trigger a immune reaction that involves macrophages entering the skin and releasing pro-inflammatory factors.

The clear areas (when present) apparently represent areas where macrophages have been cleared from the skin and thus are no longer triggering a inflammatory response.

The different patterns associated with the rashes can be explained by differences in how fast macrophages are cleared from the skin.

Reference: Vig, D. K., & Wolgemuth, C. W. (2014). Spatiotemporal evolution of erythema migrans, the hallmark rash of Lyme disease. Biophysical journal, 106(3), 763-768.


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