'lymphocyte-reactive' originates from modern English, specifically a compound of the noun 'lymphocyte' and the adjective 'reactive'. 'lymphocyte' itself comes from New Latin 'lymphocytus' (from Latin 'lympha' meaning 'clear water, spring' + Greek 'kytos' meaning 'container, cell'), while 'reactive' comes via Latin/French from Latin elements 're-' (again) + 'agere' (to do/drive) through Late Latin 'reactivus'.

'lymphocyte' developed from New Latin 'lymphocytus' into English 'lymphocyte' through 19th-century medical terminology; 'reactive' developed from Latin 'reactivus' into French 'réactif' and then into English 'reactive', and the hyphenated compound 'lymphocyte-reactive' arose in 20th-century clinical and research literature to describe substances or cells that react with lymphocytes.

Initially the parts retained their original senses ('lymphocyte' as a type of white blood cell; 'reactive' as 'showing a reaction'), and when combined the compound came to mean specifically 'causing or showing a reaction with lymphocytes' in immunological contexts; this usage is essentially technical and narrowed compared with the general sense of 'reactive'.