“…The pathogenesis of HP is complex but airborne particles that cause HP are less than 3 μm in size, a size that easily reaches the lung parenchyma (Yi, 2002). Since a small proportion of exposed persons (<10%) (Mohr, 2004) develop HP, there may be genetic predisposition of the host in addition to environmental exposure (Ando et al, 1999;Patel et al, 2001;Girard et al, 2004;Mohr, 2004;Jacobs et al, 2005). The nature of the genetic predisposition is unclear, but MHC and TNF-α polymorphism is seen in HP patients compared to normal but exposed cohorts (Camarena et al, 2001;Schuyler, 2001).…”