Interactions of surfactant protein D with pathogens, allergens and phagocytes.

Surfactant protein D (SP-D) is considered to play an important role in innate immunity in the lungs by binding, via its multiple C-type lectin domains, to carbohydrate structures present on a range of viruses, bacteria, yeasts and fungi. The resulting agglutination of the target pathogens provides host defence which can be further enhanced by killing and clearance mechanisms mediated by phagocytic cells which carry receptors for SP-D. Recent findings suggest that SP-D, and the structurally related lung surfactant protein A (SP-A), may also modulate allergic reactions by binding certain glycosylated allergens. The finding of SP-D at a variety of other sites besides the lungs, such as the gastric mucosae, is suggestive that it may play a general protective role in several secretions.