White blood cells are part of your immune system and are found in your blood and tissue. Your body has several different types of white blood cells that fight infections by producing antibodies and cytokines, killing infected cells and removing pathogenic microbes and toxins from your body. White blood cells are made from hematopoietic stem cells -- the term refers to stem cells that can convert themselves into blood cells -- that are located in the bone marrow.
Immune System
The immune system plays an important role protecting you against invading microbes, cancer cells and toxins. Your overall immune response can be divided into the innate and the adaptive response. Innate immune response is the first line of defense mechanisms mediated by serum proteins and some white blood cells, including macrophages and neutrophils. The cells and molecules of innate immunity can recognize and destroy several different particles and are not specific. The adaptive immune response is also known as specific immunity, since the cells -- B and T cells -- and molecules -- antibodies and T cell receptors -- involved recognize a specific microbe, cell or molecule and only attack it.
White Blood Cells
Eight types of white blood cells are found in your body. Phagocytic cells -- macrophages, dendritic cells and neutrophils -- engulf and destroy foreign particles. Eosinophils protect against parasites, and mast cells and basophils protect your mucosal surfaces and play a role in allergic reactions. White blood cells called lymphocytes participate in the adaptive immune response by producing antibodies -- B cells -- and killing infected and cancerous cells with T cells.
Bone Marrow
All the cells of your blood -- red blood cells, white blood cells and platelets -- are generated in the bone marrow from the hematopoietic stem cells. Bone marrow is found in the cavities of certain bones, including ribs, vertebrae, humerus, arm, pelvis and femur. All white blood cells are made from two bone marrow stem cell derivates. A myeloid progenitor cell is the precursor for neutrophils, eosinophils, basophils, macrophages, dendritic cells and mast cells, while a lymphoid progenitor cell is the precursor for B and T cells.
Development
Certain mechanisms differentiate stem cells into each of the white blood cells. First the hematopoietic stem cell will differentiate into a myeloid or lymphoid progenitor cell in the bone marrow. The myeloid progenitor cell will further differentiate into a granulocyte/macrophage progenitor cell and megakaryocyte/erythocyte progenitor cell, which gives rise to red blood cells and platelets. The granulocyte/macrophage progenitor migrates to the blood, where it differentiates into the individual white blood cells. The lymphoid progenitor cell either stays in the bone marrow and develops into B cell, or migrates to the thymus and develops into a T cell.
References
- "Immunobiology: The Immune System in Health and Disease"; Janeway, et al; 2001
- "Anatomy and Physiology"; Gary Thibodeau and Kevin Patton; 2007
