SP-A, or Surfactant Protein A, 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, the tiny air sacs in the lungs. This protein aids in the recognition and clearance of pathogens, contributing to the body's defense against infections. SP-A is produced by alveolar type II cells and bronchial epithelial cells. It binds to various microorganisms, enhancing their uptake by immune cells. This process is essential for maintaining lung health and preventing respiratory diseases, making SP-A an important component of pulmonary immunity.