White blood cells are the infection-fighting cells in the blood and tissues of the body. The white blood cell count (WBC) is an estimate of the number of white blood cells in the blood available to fight infection. When the number of white blood cells is lowered, your child has less ability to fight an infection.

There are five types of white blood cells, each one has a slightly different job. A test called a differential is used to count the different types of white cells under a microscope. The five types of blood cells are:


These cells play a major role in immunity. Most common lymphocytes include:

  • B cells
  • T cells

These cells work together to find and destroy foreign cells and body cells that do not perform well. B cells make antibodies to protect the body against infections such as measles or chicken pox. Immunizations are given to cause the B cells to create antibodies for protection.

Lymphocytes can live years or even decades, and they make up about 25% of all the white blood cells in the body. 


These cells, also called segs and bands, help fight bacterial and fungal infections. They are the first to arrive at the site of infection. They are made in a healthy body at the rate of 80 million per minute, but they are short-lived, usually only lasting 2 to 3 days. They make up about 60% of the body’s white blood cells.

When your child does not have enough neutrophils, he/she has a condition called neutropenia. For more information about neutropenia see the section on absolute neutrophil count.


These short-lived cells are involved in allergic responses. They may prevent the immune system from overreacting. They are thought to be responsible for attacking parasites.


These short-lived cells are involved in allergic and stress responses.


These cells stay in the bloodstream for about a day and then move to tissues where they become macrophages. They are the body’s scavengers. They travel to sites of infection and can swallow foreign invaders.

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