Certain white blood cells resemble unicellular organisms, particularly amoebas, in that they are free of set tissue structures, are capable of moving themselves via extension of pseudopods, and they destroy pathogens and particles by engulfing them, according to Boundless.com. These white blood cells are known as phagocytes. In the blood, these are generally monocytes and neutrophils. In other tissues, these cell types become macrophages, which are even more amoeba-like.
Monocytes and neutrophils are by far the most like single-celled organisms. Other types of white blood cells do not move much, and they serve only to mark invaders or release destructive chemicals on them. Red blood cells are even less like independent organisms, as their movement is purely passive and they lack a nucleus and thus any of the complex molecular construction independent organisms require.
Even with their nuclei, however, white blood cells still vary in important ways from single-celled organisms. The most important way is that most white blood cells cannot reproduce at all. They are generated by stem cells found in bone marrow, with new supplies being constantly generated, Boundless.com notes. These immune cells are not only found in the bloodstream. Macrophages, in particular, are found in every tissue throughout the body.