In the skin 8% of the cells are dendritic allergen receptor cells, like the one pictured to the right of the hair. These Langerhans cells engulf a molecule of haptene, like septadecylortho-benzoquinone, and crawling like a caterpillar head for the nearest lymph node, a trip that takes between four hours and a day.

At the lymph node, the Langerhans cell gives the haptene to the T4 lymphocytes. If the T4 lymphocytes have not had previous experience with that haptene, a template of that molecule is made, and a clone of memory cells with that template in the cell wall are circulated. It takes five days for the first memory cells to enter the blood stream. Additional memory cells are made for the next thirty days.

If the T4 cells have had previous experience with the haptene, memory cells are released at once, and additional cells are more quickly produced.

Memory cells leave the blood vessels repeatedly, and wander around in our tissues until they find the haptene. Then they attach to the haptene and die, killing other cells in the process, and producing the rash. Part of the skin dies, and the haptene is eliminated in the process.