means Immunology is the branch of biology and medicine that studies the immune system — how the body defends itself against pathogens, distinguishes self from non-self, and remembers past invaders.

from From Latin immunis, "exempt, free from burden" (literally in- "not" + munis, related to munus, "duty, public service") — originally a person excused from civic obligations — plus the Greek -logia, "the study of." So the word once meant exemption from taxes long before it meant exemption from disease; the medical sense grew up in the late 19th century as scientists began to understand why survivors of an illness were "exempt" from catching it again.