Friday, September 10, 2010

White Blood cells – our protectors against infections & diseases

White blood cells (WBC) or leukocytes are vital cells of the immune system protecting the human body against infections, bacteria, microbes, viruses and pathogens. These cells are produced in the stem of the bone marrow and are composed of granulocytes (Neutrophils, eosinophils, and Basophils) and non-granulocytes (lymphocytes and monocytes).
WBC’s are constantly circulating through the blood stream and once they identify a particular antigen, they immediately launch a counterattack. These cells are responsible for providing required immunity, without which it will be impossible for the body to fight against any kind of infections or diseases.

So what exactly is their function?
White blood cells act as the defending army of the human immune system providing a shield against several diseases and viruses. An inherently weak or defective immune system can lead to increased susceptibility to infection, allergies, autoimmune diseases and sometimes even cancer. These cells have a density of about 4 – 11 billion per liter of blood.
White blood cells also help produce antibodies which identify invaders and destroy body cells which have become contaminated with invading microbes. The contaminated cells survive couple of days but when they die, they are destroyed by surrounding white blood cells and replaced with new ones.
White blood cells are asymmetrical, colorless and do not contain hemoglobin. They have the ability to change their shape, which enable them to penetrate into the walls of blood vessels and between other cells. White blood cells divide by a process of mitosis, forming either more stem cells or white blood cells that can differentiate into specific white cell types, such as lymphocytes. 
Let’s find out the types of White Blood Cells
Neutrophils, lymphocytes, monocytes, eosinophils and basophils are the major types of white blood cells and each of them carry out unique functions for the immune system.  These cells play an important role in the immune system by protecting the body from various organisms and infections that contain diseases.
Neutrophils, eosinophils and basophils are collectively known as granulocytes due to prominent granules in their cytoplasm.
Lymphocytes and monocytes do not contain granules and are therefore classified under non granulocytes.
A brief description of each type of white blood cells:

Granulocytes
·         Neutrophils
Neutrophils are the most common types of white blood cell which make up 55-70% of the total white blood cells and shelter the body against disease and infections by removing and destroying bacteria, microbes, viruses, foreign substances, and other cells.  These cells play a vital role by absorbing any invading microbes that try to enter the bloodstream.
·         Eosinophils
Eosinophils are produced in the bone marrow and circulate at relatively low levels in the blood stream. About 2 – 5% of total white blood cells are comprised of eosinophils. These cells are normally found outside blood vessels in organs rather than bone marrow. A large number of eosinophils are generally found in the gastrointestinal tract. They protect the body by engulfing and killing bacteria, viruses, pathogens and other microorganisms.  Eosinophils can destroy cancer cells and secrete substances which help to stop chemicals that mediate infections and also assist the body by performing phagocytosis. Through the process of diapedesis, eosinophils can directly attack localized infections in the tissue.
·         Basophils
Basophils are the rare type of white blood cells which constitute only 1% of the total white blood cells count. They contain dark purple granules in the cytoplasm. They generally secrete biologically active substances such as histamine, proteoglycans, or cyclooxigenase products.
These cells are produced in the stem of the bone marrow. Due to the presence of IgE on their surfaces, they help release chemical mediators that cause allergic symptoms and thus play a vital role in the allergic responses.  Basophils generally circulate throughout the body in the blood stream and immediately respond to pathogens, microbes or any other infectious agent with help of other types of white blood cells. Basophils also aid phagocytosis and invoke the process of inflammation when body experiences some kind of pain.

Non granulocytes
·         Lymphocytes
Lymphocytes are the common white blood cells which detect and destroy invading viruses, parasitic worms and microbes. These cells have the ability to recognize specific invaders (antigens) and quickly launch a response to them, when they are encountered again. Lymphocytes are classified into T cells, B cells and Natural killer cells.
T Cells and B cells are specific to a particular antigen and are able to bind to a particular molecular structure. 

Generally each B cell produces one specific antibody and when it gets activated by an antigen, it produces large cells known as plasma cells which produce antibodies. The antibodies produced by the B cells will help to destroy antigens that try to damage our immune system. B cells and their antibodies, both natural and adaptive, play a fundamental role in the immediate and late defense against microbes.

T cells not only help the B cells by producing antibodies but also play a vital role in protecting our body against diseases by destroying cancerous cells and those cells which are  infected with viruses and bacteria. These cells are capable of recollecting memory against past infections and quickly expand to large number of effector T cells upon their re occurrence.

Natural Killer cells or NK cells play a crucial role in killing tumors like lymphomas, melanomas and viral infected cells such as herpes and cytomegalovirus. The functions of NK cells are similar to that of the effector cells. Lack of T and B lymphocytes can lead to severe combined immunodeficiency (SCID). These cells generally tend to get cancerous, if their count increases to a large extent. Hence the count of T or B lymphocytes should neither be too high or too low.
·         Monocytes
Monocytes are the type of white blood cells which are produced in the myelo-monocytic stem cells in the bone marrow. The size of monocytes and its nucleus are biggest among all types of WBC’s. They comprise about 3 -8% of the total white blood cell count. Monocytes are one of the major white cells that protect the body from infection and prevent microorganisms from entering the bloodstream. They also support phagocytosis and antigen processing, as a result these cells play crucial role in immune defense, inflammation and tissue remodeling. Monocytes have the ability to turn into macrophages or dendritic cells, which are vital immune system cells.

White blood cell count
White blood cell count determines the number of white blood cells present in the blood. It helps identify whether the number of white blood cells is high or low. White blood cell count is determined by the number of white blood cells per volume of blood. This test is taken according to the different types of white blood cells such as Neutrophils, lymphocytes, monocytes, eosinophils and Basophils.
 WBC count is also taken to identify presence of an infection, leukemia or any other type of disease. The WBC count is generally classified according to low white blood cell count and high white blood cell count.
White blood cell count Low
Normal white blood cell count helps the body to fight against several infections and diseases. Normally there should be 4,500 to 10,000 white blood cells in 1 micro liter of blood. But if the count of white blood cells decreases below this level it can lead to leucopenia.
Due to scarce WBC’s, body is unable to fight against invading pathogens, microbes or viruses. There are number of causes that lead to low white blood cell count.
·         Damage to the bone marrow due to certain complications can reduce the number of white blood cells.
  • Au\toimmune diseases such as Lupus react against healthy white blood cells and produce antibodies to attack it.
  • Epstein-Barr virus, tuberculosis and HIV are some of the types of viral infections that can reduce the WBC count
  • Chemotherapy not only kills cancerous cells but also healthy immune cells reducing the WBC count.
  • Aplastic anemia is one of the major causes that can reduce white blood cells to a large extent.         


     White blood cell count High
When the white blood cells exceed the normal count it can lead to leukocytosis. If the count is higher than 10,000 leukocytes in a micro liter of blood, it is considered as high white blood cell count. White blood cell count is elevated due to certain pathogens, microbes or infections. Increased white blood cell count can even lead to cancer. Postmenopausal women with elevated white blood cell counts may be at an increased risk of developing certain types of cancer, including breast, colorectal, endometrial, and lung cancers.

High white blood cell count causes
·         Acute lymphocytic leukemia is a kind of cancer which elevates WBC count as it mainly affects the bone marrow.
·         Certain use of drugs such as epinephrine and corticosteroids can increase white blood cell count.
·         Tuberculosis, acute or chronic lymphocytic leukemia, myelofibrosis and other bacterial infection can also increase the risk of high white blood cell count.
·         Smoking and stress also increase white blood cell count.
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