A peptide antigen is a short chain of amino acids that can trigger an immune response. These peptides are derived from proteins and are recognized by the immune system, particularly by T cells. When a peptide antigen is presented on the surface of a cell, it can activate T cells to identify and attack infected or abnormal cells. Peptide antigens play a crucial role in the development of vaccines and immunotherapies. By using specific peptide sequences, researchers can design treatments that stimulate the immune system to target diseases, such as cancer or viral infections. This targeted approach helps improve the effectiveness of immune responses.