Activated complement Bb and C5a fragments are increased in the plasma of HbSS mice after cold exposure. HbAA (n = 2 or 3 per group) and HbSS mice (n = 4 per group) were exposed to cold at 10°C (50°F) for 1 hour and then returned to RT or left untreated at RT. Four hours after cold exposure, mice were euthanized and EDTA blood samples were collected from the heart. EDTA plasma was run on immunoblots and stained for complement activation fragments (A) Bb, (B) C5a, and immunoglobulin G (IgG) loading controls. Zymosan-treated mouse serum, zymosan-treated heat-inactivated serum, and untreated mouse serum were analyzed on the C5a immunoblots to serve as positive and negative controls. Bb, C5a, and IgG bands on the immunoblot images were quantified using densitometry. Relative plasma Bb (C) and C5a (D) levels are presented as densities relative to IgG light chain control. Bars are means ± SEM. ∗∗P < .01 (1-way ANOVA with the Tukey multiple comparison test).