FIG. 2.

Neutralization of pertussis toxin by human serum. Pertussis toxin was incubated with increasing concentrations of human serum as indicated and added to PBMC at a concentration of 2,000 ng/ml. The cells were incubated for 3 days, and toxin neutralization was observed as production of less-reduced alamarBlue. Data are expressed as the mean ± standard deviation of at least three independent trials. An asterisk denotes a significant difference (Student t test, P < 0.05) from results for the no-toxin control wells at the same serum concentration. PTX, pertussis toxin