Main idea: Overall, as reflected in a simple complete blood count, cetirizine critically diminished immediate and late innate immune responses subsequent to allergen exposure.
IgE-mediated inflammatory responses upon allergen contact in allergic rhinitis (AR) are associated with rapid alterations in circulating blood cell numbers detectable in a complete blood count (CBC). The aim of this study was to evaluate whether the intake of antihistamines modulates allergen-induced complete blood count dynamics in male and female patients. A total of N = 112 specific allergen challenges were performed in allergic rhinitis subjects otherwise healthy. Seventy-two (n = 72) subjects received placebo and forty (n = 40) received cetirizine (H1-receptor antagonist) per os prior to allergen exposure in a randomized, double-blind trial at the Vienna Challenge Chamber (VCC). A subgroup of twenty-five (n = 25) subjects received cetirizine and placebo on different study days (parallel-group). Blood samples and symptom scores were taken at baseline and immediately after 6 h of airway challenge simulating ambient allergen contact. Female sex was associated with a pronounced circulating monocyte increase and male sex was associated with an eosinophil decrease in the placebo group, but not in cetirizine-treated subjects. The significant increase in segmented neutrophils and the decrease in circulating erythrocytes upon allergen challenge was less prominent after cetirizine intake in both sexes. A more prominent thrombocyte increase in females was noted upon allergen exposure, regardless of prior cetirizine intake. Cetirizine inhibited the mobilization of neutrophils, lymphocytes and the decline in erythrocyte numbers, but did not affect thrombocyte increase upon allergen challenge. It further diminished gender-specific blood cell dynamics.