Sialic acids, nine-carbon sugar units found at the ends of glycan chains on vertebrate and invertebrate cells, play critical roles in both health and disease. They influence physiological processes through their negative charge and hydrophilicity, modulate immune responses, and are involved in pathogen recognition. Sialic acids are also crucial for cell surface interactions, such as in the immune system, where they bind to selectins and Siglecs, affecting leukocyte adhesion and immune responses. They are present in various biological systems, including the brain, where they contribute to neuronal plasticity and ganglioside function. Sialic acids are also involved in the filtration function of the glomerular basement membrane and in the regulation of protein half-life in the bloodstream. In disease, sialic acid expression changes can be detected in pathological conditions, and their levels are used as biomarkers for various diseases. Sialic acids are also involved in the pathogenesis of infections, such as influenza, where they facilitate viral entry into cells. Additionally, sialic acid-related disorders, such as sialuria and sialidoses, are associated with genetic mutations affecting sialic acid metabolism. The non-human sialic acid Neu5Gc can cause immune responses in humans, leading to potential complications in biotherapeutic products. Sialic acids are also involved in various physiological processes, including the development of the nervous system, the regulation of immune responses, and the function of the cardiovascular system. They are present in glycoproteins and play a role in the clearance of proteins from the bloodstream. Sialic acid modifications can affect the function of mucins and other glycoproteins, influencing processes such as mucus rheology and pathogen binding. The study of sialic acids is an important area of glycobiology, with ongoing research into their roles in health and disease. Understanding sialic acid biology can lead to new therapeutic strategies for various diseases, including cancer, infectious diseases, and autoimmune disorders. The field is still evolving, with many functions of sialic acids yet to be fully understood.Sialic acids, nine-carbon sugar units found at the ends of glycan chains on vertebrate and invertebrate cells, play critical roles in both health and disease. They influence physiological processes through their negative charge and hydrophilicity, modulate immune responses, and are involved in pathogen recognition. Sialic acids are also crucial for cell surface interactions, such as in the immune system, where they bind to selectins and Siglecs, affecting leukocyte adhesion and immune responses. They are present in various biological systems, including the brain, where they contribute to neuronal plasticity and ganglioside function. Sialic acids are also involved in the filtration function of the glomerular basement membrane and in the regulation of protein half-life in the bloodstream. In disease, sialic acid expression changes can be detected in pathological conditions, and their levels are used as biomarkers for various diseases. Sialic acids are also involved in the pathogenesis of infections, such as influenza, where they facilitate viral entry into cells. Additionally, sialic acid-related disorders, such as sialuria and sialidoses, are associated with genetic mutations affecting sialic acid metabolism. The non-human sialic acid Neu5Gc can cause immune responses in humans, leading to potential complications in biotherapeutic products. Sialic acids are also involved in various physiological processes, including the development of the nervous system, the regulation of immune responses, and the function of the cardiovascular system. They are present in glycoproteins and play a role in the clearance of proteins from the bloodstream. Sialic acid modifications can affect the function of mucins and other glycoproteins, influencing processes such as mucus rheology and pathogen binding. The study of sialic acids is an important area of glycobiology, with ongoing research into their roles in health and disease. Understanding sialic acid biology can lead to new therapeutic strategies for various diseases, including cancer, infectious diseases, and autoimmune disorders. The field is still evolving, with many functions of sialic acids yet to be fully understood.