SP-D is a type of lectin, more specifically they are a collagen-containing C-type (calcium dependent) lectin which are named collectins. The collectins are responsible for immune and inflammatory control. They have a very basic structure,
* triple-helical collagen region
* C-terminal homotrimeric lectin or carbohydrate recognition domain (CRD).
SP-D is actually a monomer, these monomers assist in high affinity saccharide binding. Three of the same type of monomers associate to form a homotrimer.
SP-D has a complex quaternary structure in which monomers (43 kDa) are assembled into tetramers of trimers thus forming dodecamers. Dodecamers are further assembled into large multimeric structures. The oligomerization of SP-D results in the burial of the tail domains while the head domains are exposed. Oligomerization is dependent upon the cysteine residues at positions 15 and 20 within the N-terminal tail region.