'lymphocyte-reacting' is a modern compound formed from 'lymphocyte' + present participle 'reacting'. 'lymphocyte' comes from New Latin 'lymphocytus' (from Greek 'lymph-' meaning 'clear fluid' and 'kytos' meaning 'cell'), and 'reacting' derives from the verb 'react' (from Latin re- 'again' + agere 'to do/drive' via Romance verbs).

'lymphocyte' entered scientific English in the late 19th century from New Latin/Greek coinage 'lymphocytus'; 'react' entered English via French/Latin roots (e.g. French 'réagir') and developed the present participle form 'reacting' in Modern English. The compound construction combining a cell-name + '-reacting' is a productive pattern in 20th–21st century biomedical English.

Initially, the separate parts referred to '(clear-fluid) cell' and 'to act again/to respond'; combined in modern scientific usage they specifically mean 'producing or showing a response in lymphocytes'.