Claudins are a family of proteins that play a crucial role in forming tight junctions between cells. These tight junctions are essential for maintaining the integrity of epithelial and endothelial barriers, which control the passage of substances between different compartments in the body. There are over 20 different types of claudins, each with specific functions and properties. They help regulate permeability, allowing certain ions and molecules to pass while blocking others. This selective barrier is vital for processes such as nutrient absorption in the intestines and maintaining fluid balance in tissues.