Although the adverse health effects of diesel exhaust particles (DEP) have been proposed and are being clarified, their facilitating effects on preexisting pathological conditions (pathological conditions) have not been fully identified. On the other hand, there exist hypersensitive subjects against particulate matters. In this review, we provide insights into the immunotoxicity of DEP as an aggravating factor in hypersusceptible subjects, especially those with allergic pulmonary diseases using our in vivo experimental model. In brief, we examined the effects of DEP on allergic asthma in vivo, and showed that repetitive pulmonary exposure to DEP has promoting effects on allergic airway inflammation, including adjuvanticity on Th2-milieu. Further, we propose a causal machinery regarding the adverse impacts, i.e., via inappropriate activation of antigen-presenting cells such as dendritic cells.