The study of pathogenesis of bronchial asthma (BA) and gastroesophageal reflux disease (GERD) or their combination showed that the intensity of inflammation and the choice between Thl and Th2 immune responses are determined by macrophages (elements of congenital immunity). Lung surfactant protein D (SP-D) existing in various oligomneric forms (as monomer trimer, dodecamer, multimer) plays an important role in the mechanism of transformation ofalveolar macrophage phenotype. Patients with BA+GERD have higher SP-D level in the bronchoalveolar lavage fluid than those with GERD alone but lower than patients with BA. SP-D dodecamers were found only in BA patients given basaLtherapy with inhaled glucocorticoids (IGC). It suggests that the presence of dodecamers in the lavage fluid may result firom anti-inflammatory action of IGC. They are absent in patients with BA+GERD treated with IGC probably because GERD enhances inflammatoly changes in the lungs of BA patients despite basal therapy These data together with results of experimental acidification of lavage fluid from BA patients give reason to hypothesize that microaspiration of acidic gastric contents frequently associated with GERD is a cause of local decrease of pH in different segments of the bronchial tree triggering two pathogenetic mechanisms: (I) programming proinflammatory MI phenotype of alveolar macrophages, increased production of nitric oxide, nitrosation of SP-D and destruction of its anti-inflammatory multimers ; (b) direct destruction ofSP-D oligomers in the acid medium. Both mechanisms reduce the level of anti-inflammatory SP-D multimers and increase the level ofproinflammatory monomers. Thus, decreased pH in lower airways is a real pathogenetic factor of anti-inflammatory shift in the oligomeric SP-D composition accounting for the inflammatory reaction of lungs in GERD.