White blood cells (also called leukocytes) are diverse cell types of the immune system that are specialized for specific functions. There are two major groups of white blood cells: phagocytes and lymphocytes. Phagocytes are large cells that engulf pathogens and other substances by phagocytosis. Some phagocytes are involved in both innate immunity (the first and nonspecific line of defense against pathogens) and adaptive immunity (a slower to develop, but longer-lasting defense against a specific pathogen). In particular, macrophages and dendritic cells play key roles in communicating between the innate and adaptive immune systems. Lymphocytes include B cells and T cells, which are involved in adaptive immunity; and natural killer cells, which are involved in both innate and adaptive immunity.
The diverse white blood cells of the immune system are found in blood as well as in other tissues of the body. One milliliter of human blood typically contains about 7 million white blood cells (compare to 5 billion red blood cells). Although white blood cells circulate in the blood and attack invaders there, they also have the capacity to leave the circulatory system and enter extracellular spaces, where foreign cells or substances may also be present.
These white blood cells are also found concentrated in a variety of organs. For example, the lymph nodes and spleen contain large numbers of B cells, T cells, and macrophages. Packed in these organs, the lymphocytes and macrophages can efficiently recognize pathogens, interact with each other, and become activated to defend the body.
Textbook Reference: Concept 39.1 Animals Use Innate and Adaptive Mechanisms to Defend Themselves against Pathogens