An epitope (also known as an antigenic determinant) is a biological structure or sequence, such as a protein or carbohydrate, which is recognised by an antibody as an antigen. Recognition of the antigen occurs when an appropriate structure is formed in an area of a protein or polysaccharide in which amino acids or sugars are arranged linearly. Most proteins usually have several kinds of epitopes. The antibody offers an important technique (termed an immunoassay) for identifying specific cellular components (proteins, lipids, carbohydrates, etc.) to track the function, distribution, and modification of the protein of interest within living and fixed cells.