Homotetramer consisting of disulfide-linked alpha subunits, homooligomer consisting of disulfide-linked alpha subunit homodimers, or heterotetramer of two alpha and two beta subunits formed by non-covalent association of two disulfide-linked heterodimers (By similarity). Interacts with MBL2 through its carbohydrate moiety. This interaction may inhibit its catalytic activity.