Photomicrograph of lymphocyte proliferation in response to control (A) and mPEG-derivatized (B: 1.2 mmol/L mPEG/4 × 10⁶ PBMC) stimulator PBMC. In (B), the PBMC remain small and rounded, a morphological feature characteristic of resting, nonproliferative lymphocytes, whereas in (A), the PBMC are larger and exhibit stellate protrusions typical of antigen-stimulated cells. Shown is the response of donor 4 to donor 1 (stimulator).