The effect of loss of histamine signaling in particulate matter–induced thrombin generation. Fine urban particulate matter (National Institute of Standards and Technology Standard Reference Material, SRM 1649a 200 μg/mouse in 50 μL PBS) or vehicle was administered intratracheally to 20-25 g, 6-8 week old male C57BL/6 mice as previously described.⁴  (A) After 24 hours BAL fluid was obtained and histamine levels were measured using a commercially available assay (EIA Histamine IM2015, Beckman Coulter). (B) Mice were treated with famotidine (10 mg/kg) and desloratadine (10 mg/kg) in 150 μL of PBS 4 hours before treatment with PM followed by an additional dose of famotidine 8 hours later. Twenty-four hours after PM administration, BAL fluid was obtained and IL-6 levels were measured as previously described (ELISA).³  (C,D) H1R and H2R receptor double knockout mice (H1R^−/−/H2R^−/−) or littermate controls were treated with PM and 24 hours later IL-6 and TAT were measured in BAL fluid and citrated plasma as previously described.³  The protocol for the use of mice was approved by the Animal Care and Use Committee at Northwestern University. N = 4 or 5 animals for each group. *P < .05 compared with PBS control; and NS, not significant using ANOVA with Bonferroni posttest comparison.