The Effect of Particles on Allergic Immune Responses

In recent decades, an increased prevalence of allergic conditions has been observed in developed countries (Howarth, 1998). The development of allergic diseases depends upon both genetic predisposition and the environment. Genetic alterations, however, are not sufficient to explain the observed increase in the prevalence of allergy and asthma since these changes have occurred rapidly, over only a few decades, in very large outbred populations (Hopkin, 1997). Thus, environmental factors appear to play an important role. Both epidemiological and laboratory studies have shown that airborne pollutants, such as particulate matter, sulfur dioxide, nitrogen dioxide, and ozone, may be capable of causing adverse effects on respiratory health (reviewed in Anderson, 1997; Devalia et al., 1994; Jorres et al., 1996: Molfino et al., 1991; Nicolai, 1997; Nightingale et al., 1999; Siegel et al., 1997: Studnicka et al., 1997; Timonen and Pekkanen, 1997). In this review, we will limit our discussion to particles. However, in the real-life situation, environmental gases and particles are found in a mixture, and there are studies demonstrating that there may be a close interaction between different pollutants. These interactions may modulate the effect from the single pollution component (Adamson et al., 1999; reviewed in Gerrity, 1995 and Schlesinger, 1995). Experimental studies investigating the effect of particles on allergy-related immune responses have used different types of material: (1) mixtures of different particle types (e.g., total suspended particulate matter; Li et al., 1997; Ormstad, 2000; Takafuji et al., 1989); (2) single particle suspensions (natural or model particles; Diaz-Sanchez et al., 1999; Fujieda et al., 1998; Gilmour et al., 2000; Granum et al., 2001a; Imrich et al., 2000; Lovik et al., 1997; van Zijverden et al., 2000); or (3) solutions containing chemical substances often found adsorbed onto environmental particles (extracts of a given particle type or chemical solutions containing one particular chemical; Bommel et al., 2000; Dreher et al., 1997; Fahy et al., 2000; Gilmour et al., 2000; Lambert et al., 2000; Takenaka et al., 1995; Tsien et al., 1997). The main purpose of this article is to review studies on the effect of the particle core per se (that is the particle by itself), physical particle properties (e.g., size, number, surface area, dose-weight), and particle-bound chemical substances and metals on allergy-related immune responses. Allergy-related immune responses are also called Th2 (T helper 2) dependent, in contrast to Th1 dependent responses. The effect of particles on sensitization and provocation phases will be discussed, and the influence of the genetic background for the effect of particles will be reviewed. Some possible mechanisms behind the effect of particles will be considered briefly. However, a detailed discussion on this subject is beyond the scope of this article.