CELLS
Those cells which go on to become thymocytes migrate to thymus gland to mature i.e. fully develop. After maturing, they migrate via blood and lymph to the secondary lymphoid organs which are the sites of lymphocytes activation that take place through interaction of surveillance cells with antigens; similar to a headquarter from which orders for action are given.
T lymphocytes generate cell-mediated immunity. Cell-mediated immunity is an immune response that does not involve administration of chemicals or bodily biological bombs or poison but actual deployment of other soldiers like macrophages to the site of infection. T lymphocytes called cytotoxic T-cells directly attack invading pathogens. They also destroy infected body cells and cancerous cells.