William Hildebrand characterizes the number and nature of peptides presented by the class I human leukocyte antigens (HLA) of cancerous and infected cells. HLA class I molecules are responsible for conveying the internal health of human cells to the immune system. Class I HLA accomplish this task by first sampling a peptide from virtually every protein within a cell and then displaying these peptides at the plasma membrane. At the cell surface, T lymphocytes review the peptide cargo presented by class I HLA, with viral and cancer-specific peptides triggering T cell destruction of unhealthy cells.