White blood cells, also known as leukocytes, are a vital part of the immune system that protect the body from infection. They circulate through the bloodstream and tissues, responding to injury or illness by attacking any unknown organisms that enter the body. White blood cells are produced in the bone marrow and stored in the blood and lymph tissues. There are different types of white blood cells, each with different roles to play in defending the body against bacterial, viral, fungal, and parasitic infections. The main types of white blood cells are:

• Neutrophils and monocytes, which mainly fight against bacteria.
• Eosinophils and basophils, which are mainly involved in allergic responses.
• Lymphocytes, which can be T-cells or B-cells, produce antibodies against foreign organisms and are especially involved in fighting off viruses.

When the body is in distress and a particular area is under attack, white blood cells rush in to help destroy the harmful substance and prevent illness. White blood cells also remember how to fight the disease, and they are involved in allergic reactions. An abnormally low or high number of white blood cells might indicate an underlying problem.