Q. I had a question about PPD (Mantoux test). I read your hypersensitivity path bites and posts explaining the different types and examples of hypersensitivity. My question is: why don’t we get “sensitized” when we get our first PPD like we do with a killed or recombinant vaccine? PPD is a protein so I would expect a full immune response with memory which would cause all subsequent PPD test to be falsely positive.

A. This is a great question. It shows you’re really thinking about the material and understanding it, instead of just memorizing! Good for you. I’ve actually had the same question before, and more than once. Here’s another student who asked the same question in a slightly different way:

I was looking over some notes about the Mantoux test this weekend and I’m still not quite sure I understand it completely.  I’m confused as to why this test works more than just the first time… it would seem to me any further exposure to tuberculin after the first would be considered a “rechallenge” event, going with the logic that the first exposure (first Mantoux) would trigger events leading to a humoral response in all the following tests.  And yet this doesn’t seem to occur.  Is there a known explanation for this?

The answer is actually almost disappointingly simple: the amount of antigen injected in the Mantoux (PPD) test is too small to cause a sustainable memory response. That’s it! So the next time, it’s like your body never saw the antigen. That’s why you can do the Mantoux over and over without sensitization.