Human antibody response to urease. Results of ELISAs measuring serum IgG to purified recombinant urease C in serum samples from adults with COPD who experienced exacerbations due to H. influenzae. Patient numbers (N = 18) are noted on the X-axis. The per cent changes from pre exacerbation to post exacerbation are shown on the Y-axis. The cutoff value (dotted line) for a significant increase in antibody level was determined by averaging the difference between 8 control pairs of sera from patients who had negative sputum cultures and were clinically stable (see text).