An opsonin is a naturally occurring molecule that binds to bacteria, viruses, and other foreign particles. It is a critical part of the body’s innate immune system and plays an important role in the recognition, capture, and ingestion of these foreign invaders by immune cells. Opsonins are typically proteins that are produced by the immune system, as well as from dietary sources, such as eggs and fish. Opsonins work by increasing the chance that invading bacteria, viruses, and other microbes will be targeted and destroyed by the body’s own immune cells, helping to prevent infection and illness. In addition, opsonins also have been shown to improve the effectiveness of vaccines.