C-type lectins with a collagen-like structure inside the molecule are called collectins, and there are six molecules in humans (MBL (MBP), SP-A, SP-D, CL-L1 (Collectin-10), CL-K1 (Collectin-11), CL-P1 (Collectin-12)) [1]. In the hagfish, an ancestor of vertebrates, there are over 60 ancestral genes, which have converged into 6 genes in humans through evolution [2]. Pulmonary collectins (SP-A, SP-D) are collectins that appeared with the emergence of terrestrial animals, and they lack complement activation ability and induce microbial phagocytosis, with opsonin activity being their main biological function. On the other hand, MBL, which was first discovered, and other collectins, CL-K1, CL-L1, and CL-P1, all have complement activation ability as their primary function, recognizing cell surface glycans in organisms and foreign bodies and then triggering the activation of the complement system, which is an important function. It is then thought to work for biological defense by more efficiently opsonizing and heading toward phagocytosis and by directly destroying foreign bodies [1]. Furthermore, in innate immunity, it has been clarified that these collectins recognize the repeated glycan patterns of microbes, and the concept of pattern recognition in innate immunity, which is different from acquired immunity, has been proposed [3]. In recent years, a deficiency syndrome (3MC syndrome) caused by genetic abnormalities of either CL-K1, CL-L1, or MASP (MBL-dependent serine protease 3) has been discovered in humans, and it has been clarified that the complement activation induced by these collectin molecules is closely involved in human individual development [4] (Fig. 73.1).