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 accompanying interactive chart describes the functions, and presents micrographs, of the major cell types in the immune system.
A B cell differentiates to form plasma cells, which secrete antibodies; and memory cells, which provide immunity to future infections by a familiar pathogen.
T-helper cells assist both the cellular and humoral immune systems by secreting cytokines. Cytotoxic T cells recognize and kill virus-infected cells and other altered-self cells.
Natural killer cells attack and lyse virus-infected or cancerous cells of the body.
Monocytes circulate in the bloodstream, migrate into other tissues, and differentiate into macrophages. M
Macrophages are located in a variety of the body's tissues. They are phagocytic cells that engulf and digest microorganisms. They also activate T cells by releasing cytokines.
Dendritic cells are located in a variety of the body's tissues. They are potent antigen-presenting cells that present antigens to T cells.
Neutrophils respond rapidly to inflammation, moving from the blood into the inflamed tissue, where they phagocytize debris and pathogens. Neutrophils also phagocytize antibody-coated pathogens.
Eosinophils migrate from the blood to other tissues and kill antibody-coated parasites.
Along with MAST CELLS, basophils release histamine, which plays a major role in inflammation and in allergic response. Basophils also may promote the development of T cells.
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.