system
Given the following post, respond as if you're commenting on Reddit:
human
ELi5: Why does it take a poison ivy rash a day or so to show up and why does it take so long to go away?
gpt
The rash is an immune response to a foreign substance, but it's not just the urushiol. The urushiol soon binds to skin proteins and it is that complex that sets off the body's defense mechanism. This is why vigorous scrubbing that not only removes the oil but the skin cells bound to it is needed to prevent the rash. The rash is caused by the migration of immune cells to the region resulting in induration. It's actually the same response that is seen in positive PPD skin test for TB.

The delayed rash is the result of the time it takes to get the complicated immune response together, for the same reason that a PPD skin test takes days to show a positive result.

Since the rash is an immune response, the body has a "memory" of it so every exposure to  the **Demon Plant from Hell** results a more intense response.  


Edit changed to the possessive of "body"