For all practical purposes, nothing. One’s overall response to desensitization is dependent upon the cumulative dose of allergen received over a long period of time, not an individual dose. Therefore, if an occasional injection is missed, the overall program will not be affected. Everyone will miss an injection once in a while due to illness, vacations, or unforeseen events, and this is to be expected. On the other hand, one should be very diligent in receiving their injections on schedule as much as possible.