Caption

Cells of immune system. Coloured scanning electron micrograph (SEM) of a macrophage (blue) & lympho- cyte (white) in a lymph node. These cell types comprise the human immune system, and both are derived from white blood cells. The large macro- phage (or monocyte) has an irregular shape with thin projections called filopodia which are used for support and motility. Macrophages engulf and digest foreign bodies; they wander across tissues to sites of infection and are the first immune response. Lymphocytes (T-cells or B-cells) occur in the blood; they recognise foreign antigens and destroy invaders such as viruses.