Thicker than water

close-up view of a clot forming in human blood shows beautifully how the different blood components are crammed into the plasma. (The scanning electron micrographs have been coloured to emphasize the different structures.) Red blood cells (red) are entangled with the fibrin (yellow) that makes up the scaffolding of the clot. The platelets (blue) — which initiate clotting — are fragments of larger cells called megakaryocytes. The inset shows the rough surface of a T lymphocyte (orange), which can kill infected cells or activate other cells of the immune system.