Leukocytes or white blood cells are cells of the immune system, the body's main defense system. Their role is to defend the body against foreign materials and infectious diseases. A healthy immune system is the result of a well-controlled interplay of the different leukocytes.
Leukocytes are classified into two groups: granulocytes, which contain the granules that are filled with enzymes, and agranulocytes, which don't. Granulocytes include neutrophils, eosinophils, and basophils, and agranulocytes include lymphocytes and monocytes.
Neutrophils
In the infected tissues neutrophils digest and destroy the intruders, such as bacteria or viruses, using enzymes from their specific granules, in the process of "cell eating" or phagocytosis. Since bacteria are always present in our throat, nose, and colon, neutrophils never stop working; they check which bacteria are good, and which are bad and need to be removed.
Eosinophils
Eosinophils serve in the defense against parasites. Their level in the blood is normally low, as most are found in tissues. When activated, eosinophils release toxic granule proteins, which kill microorganisms and parasites, but can also cause tissue damage in allergic reactions. Eosinophils activation induces the synthesis of chemicals that recruit more eosinophils and other leukocytes during inflammation.
Basophils
After antibodies have neutralized bacteria or viruses that caused the infection, basophils work to remove those neutralized or dead invaders from the body. During inflammation basophils release the content of their granules, which can cause allergic reactions.
Monocytes
Monocytes mature into macrophages--large "cell eating" cells that digest and destroy foreign material or dead cells of the body. They are the first cells that encounter pathogens--disease-causing microbes--in the tissues. Macrophages have lysosomes, or spherical bodies that contain enzymes and proteins that are involved in response against pathogens. Pathogens can also activate tissue macrophages to release specialized chemicals called cytokines that start an inflammation and recruit other leukocytes to the site of infection.
Lymphocytes
Two major types of lymphocytes are B and T lymphocytes, or cells. B cells make antibodies when activated. T cells are divided into two groups: cytotoxic T cells, which kill cells infected with viruses, and helper T cells that activate other cells of the immune system, including macrophages, neutrophils, B cells, and cytotoxic T cells.
Lymphocytes can create a specific immune response against almost any foreign molecule--a so-called antigen. This is possible because during maturation each individual lymphocyte develops a unique receptor that can recognize a specific antigen. Therefore they are the main players of the adaptive immune system, which provides the immune system with the ability to remember specific pathogens. As a result, each time the body encounters these pathogens it will recognize them and launch even stronger attacks.
Leukocytes as Disease Indicators
The number of leukocytes in the blood can be an indicator of a disease. A normal amount of leukocytes in the blood is 4 to 10 billion white blood cells per liter of blood. A raised white blood count, or leukocytosis, can indicate an infection, inflammation, anemia, allergic reaction, or stress. An increase of immature or abnormal white blood cells is one of the indicators of leukemia. A low count, or leucopenia, can be due to chemotherapy, radiation-therapy, disease of the liver or spleen, or immune system disorders, including AIDS.
References
- "Immunobiology", Dr. Charles A. Janeway; Paul Travers, Ph.D.; Mark Walport, Ph.D.; Dr. Mark Shlomchik, Ph.D.; 2001
- "Kimball's Biology Pages"; Blood; John W. Kimball Ph.D.; 2010
- National Institutes of Health Medline Plus: White Blood Cell Count
