False-colour scanning electron micrograph of the cortex of a thymus. The spheres are T-lymphocytes, white blood cells vital to the cell mediated resp- onse of the immune system. Their functions include tracking down & destroying antigens directly and summoning & directing the scavenging macrophages. The thymus is arranged in two distinct zones; the outer cortex and an inner medulla. Progenitor T cells are thought to arise in the bone marrow and seed the outer cortex of the thymus. The cells proliferate giving rise to immature T cells which become immunocompetent as they migrate from cortex to medulla. Magnification: x1000 at 6x7cm size.