Could being underweight lower your immune system?

Underweight: Yes. If you have an underlying illness that makes you underweight, this could affect your immune system. If you are malnutritioned, have a poor diet, or have anorexia, and this is the cause of the weight issues, then it certainly can affect immunity. Being thin with a low body mass index, if you are otherwise healthy, does not indicate an immune problem otherwise.

Only if: If you are underweight from doing too much exercise, your immune system may be impaired due to over-exertion. Marathoners have been found to be more susceptible to respiratory tract infections. The answer is moderation. Otherwise, it should not impact your immune system unless you are malnourished.