The lectin activity of SP-A is decreased after the nitric oxide-dependent nitration of tyrosine residues [82] , and nitration decreases the ability of SP-A to enhance the adherence of Pneumocystis to alveolar macrophages [83] . However, similar findings have not yet been reported for SP-D. Conditions of mildly acidic pH, as might be found in endocytic compartments, are predicted to disrupt the lectin-dependent activities of SP-D [34]. Proteolytic degradation remains an important possibility. However, SP-D is highly resistant to degradation by a wide variety of neutral proteases in vitro, and degradation products have not yet been shown to accumulate under pathological conditions in vivo.