Final answer:
Contact dermatitis is caused by direct skin contact with irritants or allergens, leading to a type IV hypersensitivity reaction mediated by T cells, resulting in symptoms like redness and itching.
Step-by-step explanation:
The pathophysiology of contact dermatitis involves direct skin contact with irritants or allergens that trigger an immune response. This condition is classified as a type IV hypersensitivity reaction, which is a delayed response mediated by T cells. Contact dermatitis can occur upon exposure to substances like poison ivy, nickel, or latex, where the individual has typically been sensitized during a previous exposure. Upon subsequent contact, memory T cells get reactivated, leading to the production of inflammatory cytokines that stimulate macrophages and cytotoxic T cells to induce an inflammatory lesion. This results in symptoms such as redness, itching, and swelling at the site of exposure.