There are two main types of leukocytes—granulocytes and agranulocytes— cells of the human immune system differentiated based on their appearance when stained and viewed through a microscope.
Granulocyte White Blood Cells
Granulocytes have granules throughout their cytoplasm that, depending on the type, absorb stains that result in a grainy imparted color when stained.
Eosinophils are granulocytes identified using the dye eosin, with granules staining a red to orange hue. This type of white blood cell is a phagocyte, meaning that, like an amoebae, the cell can extend pseudopods, enclosing, engulfing and digesting pathogens, similar to a Pac Man eating ghosts. Eosinophs are also involved in fighting parasitic worm (nemoatode) infection.
Neutrophils are large granulocytes that stain lilac with a combination of basic and acidic dyes. Like eosinophils, these granulocytes are also able to phagocytize invading pathogens.
Basophils, the least common granulocyte, stain blue with the use of methylene blue dye. These granulocytes, along with mast cells, release histamine, a chemical that stimulates vasodilation (expansion of blood vessels) and elements of allergic response, such as mucous secretion and itching.
Agranulocyte White Blood Cells
Agranulocytes are white blood cells that do not show granules in their cytoplasm when stained. There are two types of agranulocytes—monocytes and lymphocytes.
Monocytes mature into macrophages, another type of phagocytic white blood cell that engulf and destroy invaders such as bacteria, fungi, and dust as well as cells of the body that have died.
Whereas all leukocytes previously listed are part of the second line of nonspecific immune defense, lymphocytes are white blood cells that, in addition to being involved in the nonspecific, innate immunity that we are born with, are also involved in specific immunity, the aspect of immune function that is acquired over time and exposure to the environment.
Lymphocytes include:
- NK (natural killer or cytotoxic T cells) of nonspecific immunity that can extracellularly kill virally infected cells and other abnormal body cells by secreting toxins.
- B cells are lymphocytes that either secrete specific antibodies in response to an invading antigenic particle (plasma cells) or that remember invaders so that plasma cells can respond to invasion by the same pathogen more quickly if it is encountered again in the future (memory cells).
- Helper T cells assist in regulating the activity of both killer Ts and B cells.
To learn more about the history of immune science, see the Suite101 article "Early History of Immunology."
Sources
Bauman, R. (2007) Microbiology with Diseases & Taxonomy. Pearson Benjamin Cummings.
Becker, W., Kleinsmith, L., Herdin, J., Bertoni, G. P. (2009) The World of the Cell, Seventh Edition. Pearson Benjamin Cummings.
Nester, E. et al (2001) Microbiology: A Human Perspective. McGraw Hill.
Tortora, G., Funke, B., Case, C. (2010) Microbiology, an Introduction. Benjamin Cummings.
Tami Port - Tami Port is a college professor of cell and microbiology and creator of ScienceProfOnline.com, a free science education website.
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