Immune cells are important cells that play many functions in the body: defending against pathogens and repairing tissue after injury. Perhaps the most well-known function of the immune cells is its protection against pathogens. Each day, we breathe in about a million microorganisms into our lungs, including fungi, bacteria, and viruses, and this number can be much higher depending on the environment we are in. As such, immune cells are vital in promoting rapid clearance and removal of these pathogens before they can cause an infection.