T-cell proliferation in response to tetanus toxoid challenge is decreased after mPEG-derivatization. However, the loss of proliferation is significantly less than that observed after exposure to disparate MHC-Class II antigens (see Fig 1; eg, ≈ 60% v>95% at 1.2 mmol/L mPEG per 4 × 10⁶ PBMC). Shown is the mean ± SEM tetanus toxoid-dependent proliferation of pegylated PBMC relative to control cells of 4 independent experiments.