Hereditary hemorrhagic telangiectasia (HHT) is characterized by mucocutaneous telangiectases and visceral vascular malformations. Individuals suffering from HHT have a significantly increased risk of bacterial infections, but the mechanisms involved in this are not clear. White blood cell subpopulations were estimated with flow cytometry in 79 patients with HHT and 45 healthy individuals, and association with clinicopathological status was assessed. A prominent decrease in absolute numbers of T cells in HHT was revealed (0.7 (0.5–1.1) vs. 1.3 (0.8–1.6), 106/mL, p < 0.05), and in multivariate regression analysis, hemoglobin level was associated with lymphopenia (OR = 0.625, 95% CI: 0.417–0.937, p < 0.05). Although no changes in absolute numbers of neutrophils and monocytes were observed, we revealed a significant impairment of neutrophil antibacterial functions in HHT (n = 9), compared to healthy individuals (n = 7), in vitro. The release of neutrophil extracellular traps (NETs) against Pseudomonas aeruginosa MOI10 was significantly suppressed in HHT (mean area per cell, mm2: 76 (70–92) vs. 121 (97–128), p < 0.05), due to impaired filamentous actin organization (% of positive cells: 69 (59–77) vs. 92 (88–94), p < 0.05). To conclude, this study reveals the categories of patients with HHT that are prone to immunosuppression and require careful monitoring, and suggests a potential therapeutic strategy based on the functional activation of neutrophils.