'immune-reactive' originates from Modern English, specifically formed as a compound of the adjective 'immune' and the adjective 'reactive' (the compound itself is a 20th-century medical formation). 'Immune' ultimately comes from Latin 'immunis' meaning 'exempt', and 'reactive' derives from Late Latin 'reactivus' (from 're-' + a form related to Latin 'agere'/'act-' meaning 'to do/act').

'immune' entered English via Medieval/Modern Latin and Old French (from Latin 'immunis') and became the modern English adjective 'immune'. 'Reactive' comes from the verb 'react' (formed from 're-' + a stem from Latin 'agere'/'act-') with the adjectival suffix '-ive'; both elements were combined in Modern English usage to form the compound 'immune-reactive', particularly in 20th-century immunology and laboratory contexts.

Initially the parts meant 'exempt' (from Latin 'immunis') and 'acting in response' (from Late Latin roots); the compound 'immune-reactive' developed as a technical term meaning 'capable of producing or showing an immune response' and has retained that specialized medical sense.