Responses by human splenocytes subjected to Ag challenge either in vitro, in vivo, or both were measured after transfer to SCID mice. Responses to both a neoantigen (A), horse ferritin, and a recall Ag (B), TT, were measured. A. Five SCID mice in each experiment were engrafted each with 5 × 10⁶ human spleen cells from spleen NN, that had been subjected to in vitro cultivation for 0, 3, or 10 days with (%) or without (−) 1 μg/ml horse ferritin. The hu-SPL-SCID mice were boosted at day 11 with 10 μg of ferritin (+) in IFA or IFA alone (−). In experiments 7 and 8, 2.5 × 10⁶ in vitro primed cells were transferred at the indicated day, followed by a similar number of fresh, autologous cells at day 10. *, Ferritin-specific responses were measured parallel to a human anti-ferritin Ab, 21-1B-9, and are recorded as micrograms per milliliter of this antibody, resulting in similar responses (OD₄₉₀ readings). Data are given as means ± SD. Sera were taken 29 days after the start of the experiment. B. A compilation of five experiments, with four different spleens. A minimum of four mice was used in any group in all experiments. Each SCID mouse was engrafted with ∼2 × 10⁷ cells that had been cultivated in vitro with (+) or without (−) optimal amounts of TT (see Materials and Methods). Seven days later, the resulting hu-SPL-SCID mice were then immunized with TT emulsified in alum (+) or with PBS emulsified in alum (−). TT-specific responses are reported in log₃ titers (3 was the dilution factor) as geometric means ± SD. p values were calculated based on the geometric means ± SD. †, Dilutions were started at 1:100; therefore, nonresponders have titers below 100 = 4.192 log₃. NA, not applicable.