It typically includes blotchy red spots, often with overlying firm, yellow-white bumps or pus-filled boils.

[5] There are no other symptoms associated with erythema toxicum neonatorum, and the rash does not have any long-term effects on the skin.

[5] The rash has a variable appearance, ranging from a few blotchy red spots to many yellow-white bumps and boils.

The classic presentation is 1–3 mm, firm, yellow-white bumps with a surrounding red halo.

[5][6] Erythema toxicum neonatorum is related to activation of the immune system, but its exact cause is unknown.

Many inflammatory factors have been detected in erythema toxicum neonatorum lesions, including IL-1alpha, IL-1beta, IL-8, and eotaxin.

[10] Eosinophils and other immune cells are found in the upper layer of the skin in erythema toxicum neonatorum lesions.

The leading hypothesis about the cause of erythema toxicum neonatorum is that bacteria activate the immune system when they enter hair follicles for the first time.

[4][9] This is part of a normal process in which bacteria from the environment start to grow on a baby's skin.

It is unknown whether the immune response that causes erythema toxicum neonatorum is helpful to the baby.

A health professional may make a small cut into a pus-filled lesion and collect a swab of pus for testing.

[7] The name erythema toxicum neonatorum was first used by Dr. Karl Leiner in 1912 because he believed that the rash was caused by enterotoxins.