It doesn't seem like immunocompromise generally constitutes a disability by that definition, without satisfying some other criteria. But I'm not a lawyer, so what do I know.
The text of the law makes it more explicit: "[...] a major life activity also includes the operation of a major bodily function, including but not limited to, functions of the immune system [...]"
https://www.law.cornell.edu/uscode/text/42/12102