How the Body Fights Bacteria
Your body uses white blood cells to fight off the bacteria and viruses that invade your body and make you sick. In the gif below you can see a white blood cell called a neutrophil chase down a Staphylococcus aureus bacterium. The circular cells that the white blood cell is moving through are red blood cells.
Your body is constantly covered (inside and out) by bacteria. Some are good for you, some are harmless and some can make you sick. Staphylococcus aureus is frequently found on our skin and respiratory tract. It doesn't always cause disease, but can infect the skin or lungs. There are more bacteria in and on your body than there are people on the Earth. Think on that one for a few minutes.
The white blood cell is attracted to the bacteria because proteins called antibodies have marked the bacteria for destruction. These antibodies are specific for disease-causing bacteria and viruses. When the white blood cell catches the bacteria it goes about "eating" it in a process called phagocytosis.