It starts with the penetration of allergenic substances into the outer layer of the skin. The process lasts from four days to three weeks. During this period there are no signs of skin damage. Once penetrated, the allergenic substance combines with natural skin proteins. The combination formed by the allergenic substance and skin proteins is then carried throughout the body by white blood cells called lymphocytes.
Lymphocytes are part of the immune system which guards the body against germs or alien substances.
When sensitized workers are re-exposed, lymphocytes recognize the allergen and react with it. But they also release tissue-damaging chemicals called lymphokines. These cause itching, pain, redness, swelling, and the formation of small wheals or blisters on the skin. This inflammation is usually confined to the site of contact with the allergen, but in severe cases it may spread to cover large areas of the body. It usually starts within twelve hours from exposure and is at its worst after three or four days.