The larger pale cell visible in these images is a white blood cell, while the smaller disc-shaped cells around it are red blood cells that carry oxygen.

The surface of a white blood cell is covered in receptor proteins that act like locks, recognizing antigens from bacteria and viruses as keys that signal a threat.

Chemical gradients released by infected tissue guide the cell directly toward the source of trouble, a process called chemotaxis. Once it reaches a target, cells like neutrophils and macrophages engulf the threat and destroy it using toxic enzymes.

This continuous movement is called immune surveillance, and it runs nonstop throughout the body. The same system that catches bacterial infections also scans for abnormal cells, including early stage cancer cells. It is one of the most efficient defense networks in biology.