SP-D, or Surfactant Protein 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 recognize and clear pathogens, such as bacteria and viruses. In addition to its role in lung health, SP-D is involved in modulating inflammation and promoting tissue repair. It binds to various microorganisms, enhancing their clearance by immune cells. This makes SP-D an important factor in maintaining respiratory health and defending against infections.