Surfactant protein D, also known as SP-D, is a lung surfactant protein part of the collagenous family of lectins called collectin.[5] In humans, SP-D is encoded by the SFTPD gene[6][7] and is part of the innate immune system.[8][9] Each SP-D subunit is composed of an N-terminal domain, a collagenous region, a nucleating neck region, and a C-terminal lectin domain.[5][10] Three of these subunits assemble to form a homotrimer, which further assemble into a tetrameric complex.[5][10]
Surfactant protein D has been shown to interact with DMBT1,[11][12] and hemagglutinin of influenza A virus.[13] Post-translational modification of SP-D i.e. S-nitrosylation switches its function.[14][15][16]