An armpit rash is often caused by irritation from sweat ... of substance irritates your skin, damaging its outermost layer. It is due to irritation, not an allergy. Bunick says common causes ...

Aquagenic urticaria is a rare condition where people have an allergic ... a type of itchy rash — develop rapidly after the skin comes into contact with water, including sweat or tears.

There are many possible reasons why a rash develops on the neck, including allergies, infections, insect bites, and skin conditions. Knowing what's causing your rash before treating it is important.