Surfactant protein D (SPD) is a tetramer of collagen-like triple helices terminating in trimeric lectin domains, which serve to aggregate invading bacteria by binding to carbohydrates on the surface.  SPD also binds alpha-2-macroglobulin, which protects SPD from neutralization by proteases.

Craig-Barnes, HA., et al. “Surfactant protein D interacts with alpha-2-macroglobulin and increases its innate immune potential” J. Biol. Chem. 2010, March 5 [Epub ahead of print]