This disclosure provides peptides which bind to LAG3, such as SAPWEPLHWPEDWWQGTGEW (SEQ ID NO:1), and can be used to block the interaction of LAG 3 with other molecules such as MHC-II, FGL1, and α-synuclein. These peptides can be used for various therapeutic purposes, such as inhibiting the progression of a hyperproliferative disorder, including cancer, or inhibiting the progression of a synucleinopathy, inhibiting the progression of sepsis, inhibiting the progression of an infectious disease, and enhancing a response to a vaccine.