Ever have a run in with the wrong kind of vine?
Skin cells normally work in harmony to protect the body from infection, like an orchestra. Epithelial and keratinocyte cells line the outside of the skin. Beneath are nerves, sweat glands, hair follicles, and immune system cells including dendritic cells.
When immune cells in the skin overreact to a harmless substance, it’s called allergic contact dermatitis, or skin allergy. Common causes of skin allergies—allergens—are cosmetics or perfumes, latex, or plants like poison ivy.
These allergens contain small molecules called haptens that bind to immune system proteins on skin cells. If a dendritic cell marks this allergen as dangerous, it recruits a T cell into a new deadly program. The T cell becomes a killer T cell, a white blood cell armed with cytotoxins to blast an invader.
But it takes the cell a little while to pick up the new tune. And it must duplicate itself for a stronger reaction. That’s why skin allergic reactions can take a few days to emerge, and why the second exposure leads to a quicker reaction. The killer T cells are ready and waiting in the skin, and don’t need a dendritic cell to activate them the second time around.
When the killer T cells encounter their target allergen they release their cytotoxins, delivering a lethal hit to the unfortunate skin cell. The cell’s death destabilizes the tissue. The skin swells, itches and blisters, and can take a few days or even a few weeks to rebuild itself.
The best way to prevent another rash is to avoid the trigger entirely. But if you do come into contact with an allergen, wash it off as soon as you can, before your cells start paying the price.