What does Boxing Day have to do with boxing?
mid-15c., "free; exempt," back-formation from immunity. Cf. Latin immunis "exempt from public service, free from taxes." Specific modern medical sense of "exempt from a disease" (typically because of inoculation) is from 1881. Immune system attested by 1917.
immune im·mune (ĭ-myoōn')
Of, relating to, or having immunity to infection by a specific pathogen.
Relating to the mechanism of sensitization in which the reactivity is so altered by previous contact with an antigen that the responsive tissues respond quickly upon subsequent contact.