Chemotactic effect of IL-8 on HMC-1 cells (A) and in vitro cultured human mast cells (B). Cells were exposed to various concentrations of IL-8, GRO, or PF-4, as indicated for 4 hours. The results are expressed as the number of countable cells per 3HPF (mean ± SEM) (n ≥ 5). *P < .05, **P < .01.