'immunoreactive' originates from a combining form 'immuno-' (from Latin 'immunis' via Modern Latin meaning 'exempt, protected') combined with 'reactive' (from Latin roots 're-' meaning 'again' and 'agere' meaning 'to do', via Medieval/Modern English forms).

'immuno-' was formed in Modern Latin/Neo-Latin from Latin 'immunis' and used as a productive scientific prefix; 'reactive' developed from Latin-based 'react-' elements and the adjective suffix '-ive', and the compound 'immunoreactive' arose in 20th-century biomedical English to describe immune-system reactions.

Initially the parts referred separately to 'immune/protected' and 'capable of reacting'; over time the compound came to mean specifically 'capable of reacting with immune reagents (antibodies/antigens)' in laboratory and clinical contexts.