T-cells are created continuously in bone marrow and “educated” (selected against recognizing self-antigens) in the thymus. Afterwards, they migrate to lymph nodes and other tissues, where they patrol for cells infected by pathogens. Each T-cell recognizes a specific antigen, which in our context is an “epitope”, a short segment of an HIV protein in a complex with an HLA molecule, exposed on the surface of an infected cell. T-cells that have not encountered their cognate antigen are referred to as “naive” cells; those that have are called “memory” cells.