Causes:

Any substance that does not suit the skin and causes an allergic reaction is an irritant or allergen.

Anything from plants like poison ivy to dyes and fragrances found in everyday products might be allergens. One could also have an allergic reaction to something in the air that settles on the skin, like plant pollen, chemical sprays and powders, or cigarette smoke.
It can be caused by harmless substances like everyday clothing materials(mostly woolen or nylon clothes), certain chemicals in food, perfumes, metals used in jewellery, soaps, laundry detergents, creams, oils, fabric softeners, shampoos — or even excessive exposure to water. or sunrays.

Excessive moisture caused by sweat can accumulate in the folds of the skin and is known to be a common cause of skin irritations and allergies. Such Irritants or allergens differ as per the skin type.
People who have allergies react to substances that wouldn't bother most others. The doctor calls this allergic contact dermatitis.

Symptoms:

The affected area may show redness and swelling. There could be eruptions or red patches, a mild or severe itch and even pus formation in some cases. In case of an itch, continuous scratching of the area may also cause burning sensation.