Factor D is a protein that plays a crucial role in the immune system, specifically in the alternative pathway of the complement system. This pathway helps the body fight infections by marking pathogens for destruction and promoting inflammation. Factor D is essential for the activation of other complement proteins, which work together to eliminate harmful microorganisms. Deficiency in factor D can lead to increased susceptibility to infections, particularly from certain bacteria. Understanding factor D and its functions can help researchers develop new treatments for immune-related disorders and improve our knowledge of how the immune system operates in health and disease.