Surfactant protein D (SP-D)

Surfactant protein D (SP-D) is a key molecule in pulmonary immunity and surfactant homeostasis, with broad roles in innate immune defense, inflammation regulation, and pathogen clearance. It is a hydrophilic glycoprotein composed of twelve identical 43 kDa polypeptides, forming a large cruciform-like dodecamer. SP-D acts as a soluble pattern recognition receptor, binding to pathogens and modulating immune responses.

SP-D acts as a soluble pattern recognition receptor. Its CRDs bind carbohydrates or charge motifs on microbial surfaces. The collagenous tail interacts with receptors on immune cells—such as calreticulin-CD91 complex—to trigger phagocytosis or modulate cell signaling pathways involved in inflammation resolution or promotion depending on context. Oligomerization enhances ligand affinity.