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Given a text passage, come up with an appropriate title for it. The title should accurately represent what the main focus of the passage is. The title of a passage may be present in the first few sentences of the passage and/or be referred to multiple times in the passage. The title should be 1-5 words long.

White blood cells ( also called leukocytes or leucocytes and abbreviated as WBCs ) are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders . All white blood cells are produced and derived from multipotent cells in the bone marrow known as hematopoietic stem cells . Leukocytes are found throughout the body , including the blood and lymphatic system .
Output:
White blood cell