Sialic acid is a type of sugar molecule found on the surface of cells in many living organisms, including humans. It plays a crucial role in various biological processes, such as cell recognition, signaling, and immune response. Sialic acids are often attached to proteins and lipids, forming structures known as glycoproteins and glycolipids. These molecules are important for the proper functioning of the nervous system and can influence how cells interact with each other. Sialic acid is also involved in protecting cells from pathogens, making it a key component in the body's defense mechanisms against infections.