As a consequence of intracellular oligomerization, monomeric SP-A subunits fold into trimers, and supratrimeric assembly leads to high-order oligomers [41, 42] . The degree of supratrimeric oligomerization is important for the host defense function [14, 41, [43] [44] [45] . SP-A1 and SP-A2 differ in only four amino acids (residues 66, 73, 81 and 85) located in the collagen domain [46] . In most functions examined, recombinant human (rh) SP-A2 shows higher biological activity than SP-A1 [14, 41, [47] [48] [49] [50] .