Particulates are considered to be one of the causative factors for allergic asthma and rhinitis in developed countries. Indeed, particulates and crystals are reported to activate type-2 immunity, which is associated with allergic responses. Thus far, many studies have been carried out to determine how particulates trigger immune responses; however, the detailed mechanisms by which immune responses are triggered are still unknown. One important clue is that particulates have an adjuvant activity and boost immune responses toward type-2 responses. Most adjuvants are considered to activate innate immunity, and then activated innate immune cells stimulate adaptive immunity, which involves T cells and B cells. That is why many scientists believe that particulate adjuvants directly stimulate innate cells through unique sensor proteins such as pattern-recognition receptors. In this review, we will introduce the proposed mechanisms of particulate-induced immune activation from the viewpoint of the activation of innate immune responses and related receptors and sensors.