The immune system constantly monitors all cells in the body to identify and eliminate any cell that becomes infected or cancerous. A key component of the immune system is a transporter called TAP, which resides inside the cell on the membrane of a compartment called the endoplasmic reticulum (ER). Normally, TAP imports protein fragments called peptides into the ER, after which they are displayed on the cell surface and examined by immune system cells. If the immune system recognizes peptides derived from a virus or tumour, the cell is destroyed. In many cancer cells, TAP is present in very low levels, so viral or tumour peptides do not get into the ER for identification. As a result, the T cells that recognize and kill aberrant cells are not activated and diseased cells continue to grow. At the opposite end of the immunity spectrum, T cells may identify cells from a donor organ as being foreign to the host’s body and kill these cells, resulting in transplant rejection. Robyn Seipp is investigating whether different forms of TAP can be used to improve the immune system’s ability to recognize and respond appropriately to both tumour cells and donor cells from a transplanted organ. The results may be used to improve cancer treatments and reduce transplant rejection.