Translated from G. Toepfer, Historisches Wörterbuch der Biologie (2016), vol. 3, pp. 226–227:
(...) The discovery of a specific immune system in vertebrates took place in the 20th century.89 The immune system concept was already used sporadically in the first decades of the 20th century. Thus in 1909 E. Meyer and E. Emmerich reported their investigation of a “bacteriolytic immune system”90 and in 1920 H. Schade spoke of the “hemolytic immune system, consisting of blood cells and associated hemolysin-amboceptor”.91 But until the 1950s, the expression covered primarily laboratory investigated defense reactions, viz. the reaction system of antigens and antibodies92 — the expression only solidified into the concept of a body’s defense system in the mid-60s, first under the name lymphoid system,93 but soon thereafter as “immune system”.94
89 Cf. Moulin, A. M. (1989). The immune system: a key concept for the history of immunology. Hist. Philos. Life Sci. 11, 221–236.
90 Meyer, E. & Emmerich, E. (1909). Über paroxysmale Hämoglobinurie. Deutsches Archiv für klinische Medizin 96, 287–327: 305.
91 Schade, H. (1920). Die physikalische Chemie in der inneren Medizin: 135; cf. 2nd ed. (1923): 142.
92 Bier, O. G., Siqueira, M. & Osler, A. G. (1955). Studies on the mechanism of hypersensitivity phenomena 1. Internat. Arch. Allergy Appl. Immunol. 7, 1–9: 5.
93 Cooper, M. D., Peterson, R. D. & Good, R. A. (1965). Delineation of the thymic and lymphoid system in the chicken. Nature 205, 143; (...)
94 New York Times, 24 Oct. 1965: IV. 8/3 (after OED).