15. A researcher is analyzing the immune response of a patient following the patient's exposure to an unknown agent while out of the country. The patient's blood is found to have a high proportion of lymphocytes with CD8 surface proteins. What is the likely cause? (A) The patient encountered a bacterial infection which elicited CD8 marked T cells. (B) The disease must have been caused by a multicellular parasite, such as can be encountered in polluted water sources. (C) The CD8 proteins would be discharged from these lymphocytes to lyse the infected cells. (D) The CD8 proteins marked the surfaces of cytotoxic 'T cells to attack virus-infected host cells. (E) CD8 marks the surface of cells that accumulate after the infection is over and signal patient recovery.