A lymphocyte is a type of white blood cell that protects your body from infections and diseases. These tiny cells patrol your bloodstream and tissues like microscopic guards, searching for germs, viruses, and other invaders that don't belong in your body.

Your body contains trillions of lymphocytes, and they work in remarkably clever ways. Some lymphocytes, called B cells, produce special proteins called antibodies that stick to germs and mark them for destruction. Other lymphocytes, called T cells, directly attack infected cells or help coordinate your body's immune response. When you get a vaccine, you're actually training your lymphocytes to recognize a particular germ so they can fight it off quickly if you ever encounter it for real.

Lymphocytes have an impressive ability: they can remember germs they've fought before. This is why you usually only get chickenpox once. After your lymphocytes defeat it the first time, some of them stick around as memory cells, ready to attack immediately if that same germ ever returns.

When doctors check your blood during an illness, they often count your lymphocytes. A high lymphocyte count might mean your body is fighting an infection, while a low count could signal that your immune system needs attention.