Now, our red blood cells make up the predominant portion of the cells of our blood. Leukocytes only make up about 1% of the cells of our blood. But when we have some type of infection, this number can greatly increase. And this is because leukocytes are actually our immune cells that protect us from bacterial and viral agents. So we have different types of leukocytes and we'll focus on these different types and their functions. When we'll discuss our immune system.