You are answering a USMLE-style medical question. Question: A medical researcher is studying the physiology of the immune system in order to better understand the effects of HIV on patients. He isolates a group of cells that are shown by flow cytometry to be positive for the cell surface marker CD8. He then mixes this cell population with a group of infected cells, crosslinks extracellular interactions, and immunoprecipitates the CD8 protein. He identifies a protein bound to CD8 that is composed of two chains from an adjacent cell. Which of the following best describes the primary function of the protein that was most likely identified? Options: A) Binds complement proteins on the cell surface B) Binds endogenous peptides that are present in the endosome C) Binds endogenous peptides that have been transported by the TAP channel D) Binds exogenous peptides that are present in the endosome Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.