Functional roles of the lung surfactant proteins SP-A and SP-D in innate immunity.

The surfactant proteins SP-A and SP-D are known to play important roles in innate immunity in the lungs in the protection against a wide variety of potential pathogens, which include viruses, bacteria and fungi. Recognition of the pathogens is, in most cases, mediated by the Ca+2-dependent binding of the C-type lectin domains of SP-A, or SP-D, to carbohydrate structures on the surface of the microorganisms. This can cause agglutination of the pathogens and also possibly their enhanced killing and clearance by phagocytic cells which carry receptors for SP-A or SP-D. Both SP-A and SP-D are found in a number of other sites in the body, in various secretions, and therefore may play a wider protective role than was initially envisaged.