White blood cells are cells circulating in the blood that defend the body from foreign invaders. They are considered part of the immune system. There are five major types of white blood cells: neutrophils, eosinophils, basophils, lymphocytes, and monocytes. Neutrophils, eosinophils, and basophils are known as granulocytes; lymphocytes and monocytes are known as agranulocytes.
As the name suggests, the granulocytes contain granules—small particles that take up certain stains. Granules appear to be small grains suspended within the cell. They are actually small sacs containing enzymes that help each cell protect the body from intruders. The granulocytes are named according to their staining characteristics. The granulocytes are also known as polymorphonuclear cells, due to their multilobed nuclei.
Neutrophils are the most common type of white blood cell. These cells’ granules take up neutral dyes, and appear purple. These cells belong to a special class of cells called phagocytes. Phagocytes are cells that surround, swallow up, and digest foreign invaders. This cell is the most common leukocyte; from 50 to 60 percent of white blood cells circulating in the body are neutrophils. Neutrophils are also known as “segs” or “polys”.
Eosinophils are cells whose granules take up acidic stains and appear red. These cells are active during allergic responses, when they release histamine and other proteins. They also help the body to fight off large parasites, such as worms. They make up 3 percent of the white blood cell population.
Basophils are cells whose granules take up basic stains and appear blue. The exact function of these cells is unclear. They increase in number during the healing that takes place after inflammation. They also play a part in allergic reactions, along with eosinophils. They are the least common leukocytes; they make up about 1 percent of the body’s white blood cells.
Agranulocytes contain only a small amount of granules (generally too little to be seen under a light microscope). These cells have a single nucleus that takes up most of the space inside the cell.
Lymphocytes come in two varieties: T cells and B cells. These cells are named based on where they are formed; T cells mature in the thymus (a small gland located in the chest), while B cells are made in the bone marrow. T cells attack cancerous cells and cells infected by viruses. They also produce chemicals that assist other cells in the immune system. B cells make antibodies. These are proteins which bind to foreign invaders and help destroy them. About 32 percent of all white blood cells are lymphocytes.
Monocytes are very similar to neutrophils, in that they are also phagocytes. These cells move from the blood into tissues, and become another type of cell called a macrophage. Macrophages are also phagocytes. They also help T cells fight invaders in a process called antigen presentation, and release substances that help the immune system respond to pathogens. Around 4 percent of circulating leukocytes are monocytes.
An easy to remember mnemonic for these cells is: Never Let Monkeys Eat Bananas. In this mnemonic, the cells are arranged in order from most common (N, or neutrophil) to least common (B, for basophil).
Family Practice Notebook: Basophil Count, Monocyte Count, Lymphocyte Count
MedTerms Dictionary: Basophil, Neutrophil, Eosinophil
Davi-Ellen Chabner, The Language of Medicine.