(A) Increased endothelial adherence of sickle cells in buffer. Sickle cell and normal cell endothelial adhesiveness was measured in HBSS-BSA buffer and the figure presents the relative adherence with respect to normal controls (see Materials and Methods). (B) Direct paired measurement of the relative adherence of sickle cells in buffer and in autologous plasma. Error bars associated with the relative adherence of sickle cells represent standard deviations for the number of experiments indicated in the parentheses. By definition, there is no standard deviation associated with the normalized relative adherence of normal cells; however, in (A) and also in Figs 4 and 6 below, a dashed error bar representing the average standard deviation of the number of bound control cells in replicate wells within each experiment is provided as an additional index of internal variability independent of the statistical analysis. The asterisk in (A) indicates statistical significance at P < .001 (see Materials and Methods). In (B), the relative adherence of sickle cells in buffer and in plasma did not differ at the .05 level (sickle cell adherence in buffer or in plasma was nonetheless significantly different from the unitary adherence of control normal cells at P < .001).