Floating in your blood right now is a vast army of cells whose only job is to defend you from invaders. They’re called white blood cells (or leukocytes), and they make up about 1% of your blood. There are several types, each specialized for a different threat.
Neutrophils are the foot soldiers - the first ones to arrive at a wound or infection. They eat bacteria. Macrophages are the cleanup crew - they swallow up debris and broken cells. T cells and B cells are the smart ones - they can recognize specific invaders and either kill them or make antibodies (custom-shaped proteins designed to stick to specific germs).
The really clever part is memory. After your immune system fights off an infection, certain B and T cells stick around as “memory cells.” Next time that same invader shows up, the memory cells recognize it instantly and mount a faster, stronger response. That’s how vaccines work - they teach your immune system to remember a germ without you actually having to get sick from it.
