Leukocytes, also known as white blood cells, are the cells of our immune system that function to protect and defend our body cells from different types of pathogens, for example, bacterial cells as well as viral agents. Now, just like red blood cells, our white blood cells also come from the same exact stem cell found in the bone marrow of our body. And the stem cells cells are known as hematopoietic stem cells. Now, all white blood cells, or generally speaking, white blood cells, can actually differentiate into many different types of specialized white blood cells, as we'll see in just a moment. And all these specialized white blood cells have their own unique purpose and function. So let's begin by taking a look at the following flowchart, which basically describes the different types of categories of leukocytes.