Surfactant Protein D (SP-D) is a protein that plays a crucial role in the immune system, particularly in the lungs. It is part of the surfactant system, which helps reduce surface tension in the alveoli, allowing for proper lung function. SP-D is produced by alveolar type II cells and bronchial epithelial cells, and it helps to trap and clear pathogens, such as bacteria and viruses, from the respiratory tract. In addition to its role in lung health, SP-D is involved in modulating inflammation and immune responses. It binds to various microorganisms and enhances their clearance by immune cells, contributing to the body's defense mechanisms. Abnormal levels of SP-D can be associated with respiratory diseases, making it a potential biomarker for conditions like chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis.