Lymphocyte (Low and High Levels)






A lymphocyte is a type of white blood cell present in the blood. White blood cells help protect the body against diseases and fight infections. When the general defense systems of the body have been penetrated by dangerous invading microorganisms, lymphocytes help provide a specific response to attack the invading organisms. A microorganism is a tiny organism made of one cell that is usually too small to be seen without using a microscope. Lymphocytes help to protect the body against tumors (tissues that grow more rapidly than normal). However, lymphocytes can also cause the rejection of tissues during organ transplants because they interpret these tissues as foreign invaders.

FEATURED BOOK: Mosby's Diagnostic and Laboratory Test Reference (2012)

WHAT ARE SOME OTHER CHARACTERISTICS OF LYMPHOCYTES?

Lymphocytes are small white blood cells, usually 7 to 8 micrometers in length. A micrometer is a very small unit of length that measures one millionth of a meter. A meter is approximately 39 inches (slightly more than 3 feet). Larger forms of lymphocytes are usually about 10 to 20 micrometers in length.
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The nucleus (central structure) of a lymphocyte is made of large groupings of thin threads known as chromatin. The nucleus of a lymphocyte stains dark purple/blue when exposed to a stain known as Wright's stain. You can see what this looks like below. As you can see, the nucleus is usually round but can be slightly indented. Also, the nucleus is surrounded by a small amount of light blue cytoplasm (a gel-like substance that fills up a cell).

Unlike other types of white blood cells, such as basophils and eosinophils, the cytoplasm of lymphocytes usually do not contain large, rough-looking, grain-like particles. However, larger forms of lymphocytes may have a lot of cytoplasm that contain several bright reddish/purplish, rough-looking, grain-like particles. Unlike some other types of cells, the granules (tiny substances) of lymphocytes do not turn a blue color when exposed to certain types of chemical used in laboratory tests.

WHAT DOES A LYMPHOCYTE LOOK LIKE?

A lymphocyte looks like this under a microscope:
WHERE ARE LYMPHOCYTES MADE?

Lymphocytes are formed in lymphatic tissue throughout the body. Lymphatic tissue are a network of fibers and cells that help protect the body against disease. Examples of places in the body where lymphocytes are made that are made of lymphatic tissue include the spleen, thymus, tonsils, and lymph nodes. The spleen is an organ next to the stomach that helps fight infection and removes and destroys worn-out red blood cells. Red blood cells are cells that help carry oxygen in the blood. The thymus gland is an organ located in the upper part of the chest and is very important in producing substances that protect the body against disease. The tonsils are a pair of oval masses at the back of the throat. Lymph nodes are small egg shaped structures in the body that help fight against infection.

Lymphocytes are also made by fetal stem cells and develop in the bone marrow (a tissue that fills the openings inside of bones). Fetal stem cells are the earliest type of cells that are present in the body as a developing baby. Stem cells give rise to all of the different types of cells in the body. After the lymphocytes are formed, they are released into the circulating blood.

ARE THERE DIFFERENT TYPES OF LYMPHOCYTES?

Yes. There are two types of lymphocytes. One type of lymphocyte is known as a T cell. The other type is known as a B cell. T cells and B cells differ in function and the molecules that are on their surface also differ. A molecule is a combination of atoms. An atom is the smallest part of a substance that can exist alone or in combination with something else. See the next two sections for a description of B cells and T cells.

WHAT IS A T CELL?

T cells (also known as T lymphocytes) are types of lymphocyte that circulate through the thymus gland and have turned into cells known as thymocytes (cells that have developed in the thymus gland). The thymus gland is an organ located in the upper part of the chest and is very important in producing substances that protect the body against disease. When thymocytes are exposed to antigens (substances in the body, such as those present on the surface of bacteria, that can produce a defensive reaction by the body), they rapidly divide and produce large numbers of new T cells that are sensitive to that type of antigen. More than 80% of lymphocytes in the circulating blood are T cells.

There are two main groups of T cells. One group of T cells are called "killer cells" (also known as cytotoxic T cells) because they produce chemical substances known as lymphokines that are essential in helping the B cells (see next section) destroy foreign substances. Like B cells (as is described in the next section), T cells are sensitized and stimulated to respond to certain antigens present on invading microorganisms or abnormal cells. Another group of T cells are called helper T cells. Helper T cells assist the killer T cells in performing their activities and help protect the body against diseases in other ways.

T cells also appear to play an important role in the body's response to the spreading of cancer cells. Cancer is a group of diseases in which symptoms are due to an abnormal and excessive growth of cells in one of the body organs or tissues. The process of the T cells protecting the body is known as cellular or cell-mediated immunity.

T cells reproduce through a process known as mitosis, in which the cells split in two. In mitosis, each cell contains an exact copy of the chromosomes in the original cell. Chromosomes are structures in a person's cells that contain proteins and a substance known as DNA (an abbreviation for deoxyribonucleic acid). DNA is a chain of many connected genes. Genes are units of material contained in a person's cells that contain coded instructions as for how certain bodily characteristics (such as eye color) will develop.

WHAT IS A B CELL?

B cells (also known as B lymphocytes) are types of lymphocytes that circulates in the blood in an immature (not fully developed) form. About 10% of lympocytes that circulate in the blood are B cells. B cells produce proteins known as antibodies that are then inserted into the area that immediately surround the cytoplasm (a gooey substance that fills up a cell). Antibodies attach to foreign proteins in the body known as antigens that are found on the surface on certain microorganisms. A microorganism is a tiny organism made of one cell that is usually too small to be seen without using a microscope.

As an example of the above, some antibodies help fight against bacteria because bacteria commonly contain many antigens. When the antibodies attach to the antigens on the microorganism, this starts a process that lead to the death of the microorganism. This process of the B cells protecting the body is known as humoral immunity, because the B cells release the antibodies into the fluids (also known as humors) of the body. B cells also reproduce through a process known as mitosis (see the last section for a description). When the B cells divide, each cell has identical copies of antibodies on their surface.

Sometimes an immature B cell is exposed to an antigen that characterizes a single class of microorganisms. An example of microorganisms would be bacteria. When the B cell is exposed to such antigens, it becomes activated and travels to the spleen or the lymph nodes. The spleen is an organ next to the stomach that helps fight infection and removes and destroys worn-out red blood cells. Lymph nodes are small egg shaped structures in the body that help fight against infection. When the B cells reach the spleen and the lymph nodes under such conditions, they change into plasma cells and memory cells.

The plasma cells make and release antibodies. The memory cells do not release antibodies but they "remember" antigens they were exposed to in the past so they can deal with them in a quicker way when exposed to these antigens in the future. When exposed again to one of these antigens the memory cells turn into plasma cells and release antibodies to fight off the antigens. This is how vaccines (preparations given to protect the body against infections) work. That is, vaccines expose the body to certain types of antigens so that the memory cells can be prepared for them if encountered in the future and fight against them quickly. Memory cells can survive in the body for many years.

WHAT PERCENT OF WHITE BLOOD CELLS ARE LYMPHOCYTES?

Approximately 15% to 40% of white blood cells are lymphocytes. It is important to keep in mind that the ranges mentioned above will be different depending on the machine used to do the blood test. Always use the normal range printed on the lab report to decide what range is normal.
WHAT CAN CAUSE THE LEVEL OF LYMPHOCYTES TO BE TOO HIGH?

There are many possible causes for an abnormally high lymphocyte count (known as lymphocytosis). A lymphocyte level that is more than 40% is considered to be abnormally high. Some basic causes of a high lymphocyte count are the flu and the chickenpox. Other causes of a high lymphoocyte level include tuberculosis, mumps, rubella, varicella, whooping cough, brucellosis, and herpes simplex. A brief description of these conditions are listed below:

Tuberculosis is a type of infection that usually affects the lungs. Mumps is a type of virus that occurs suddenly, mainly in children, and usually causes the appearance of swelling in the face. Rubella is type of virus infection that affects both children and adults. Whooping cough is a type of infection that causes a cough and a whooping sound when breathing in. Brucellosis is a rare type of bacterial infection that is transmitted from animals, such as cows, goats, and pigs. Herpes simplex is a virus that usually causes flu-like symptoms and cold sores.
Acute lymphoblastic leukemia (ALL) and chronic lymphocytic leukemia (CLL) cause increases in lymphocytes. Leukemia is a type of cancer in the blood in which bone marrow (a tissue that fills the openings of bones) is replaced by early forms of white blood cells. Cancer is a group of disease in which symptoms are due to an abnormal and excessive growth of cells in one of the body organs or tissues. ALL is a type of leukemia in which there are too many immature (not fully developed) white blood cells called blasts that are produced by the bone marrow (a tissue that fills the openings of bones). One type of blast present in ALL is the lymphoblast, which is an immature lymphocyte. CLL is a type of leukemia characterized by the presence of too many mature lymphocytes.
Other conditions that can cause an abnormally high lymphocyte count include mononucleosis, hepatitis, toxoplasmosis, and cytomegalovirus. Mononucleosis is suddenly occurring virus infection that is characterized by an abnormal increase in a type of white blood cell in the blood. Hepatitis is an infection of the liver that causes liver inflammation. The liver is the largest organ in the body and is responsible for filtering (removing) harmful chemical substances, producing important chemicals for the body, and other important functions. Toxoplasmosis is a type of infection of birds, reptiles, and other animals that can also occur in humans. Cytomegalovirus is a type of virus that usually causes infections in the eye and the stomach area.

Certain medications can also lead to a high lymphocyte count. Two such medications are Dilantin and mephenytoin, which are both anti-seizure medications Seizures are involuntary muscle movements and/or decreased environmental awareness due to overexcitement of nerve cells in the brain. High lymphocyte levels also occur after blood transfusions. A blood transfusion is a procedure in medicine in which blood (usually from another person) is introduced into someone's bloodstream.

WHAT CAN CAUSE THE LEVEL OF LYMPHOCYTES TO BE TOO LOW?


There are many possible causes for an abnormally low (less than 15%) lymphocyte count (known as lymphocytopenia). Since lymphocytes are made in the bone marrow (a tissue that fills the openings of bones), if not enough bone marrow is produced or the activity of the bone marrow decreases, an abnormally low lymphocyte count can occur. Bone marrow destruction occurs in aplastic anemia. Aplastic anemia is a condition in which the there is an abnormal decrease in the cells that make up blood (such as white blood cells) because the bone marrow is not working properly to make these cells. Aplastic anemia can cause an abnormally low lymphocyte count.

People with AIDS (Acquired Immune Deficiency Syndrome) often have an abnormally low lymphocyte count. AIDS is a decrease in the effectiveness of the body's immune (defense) system that is due to infection from a virus known as HIV (human immunodeficiency virus). Tumors (tissues that grow more rapidly than normal) can cause an abnormally low lymphocyte count.

Steroids can cause an abnormal decrease in the lymphocyte account. Steroids are any of a large number of hormone substances with a similar and basic chemical structure. Hormones are natural chemicals produced by the body and released into the blood that have a specific effect on tissues in the body. An overactive adrenal cortex can also cause an abnormal decrease in the lymphocyte count. The adrenal cortex is the outer part of an important organ (the adrenal gland) that produces hormones that are essential for the body to regulate itself.

Some disorders that affect the nerves in the body can cause the lymphocyte level to be too low. Examples are multiple sclerosis, myasthenia gravis, and Guillain-Barre syndrome. Multiple sclerosis is a condition in which people develop multiple areas of abnormal patches (also known as plaques or sclerosis) in the brain and/or spinal cord (depending on the stage of the illness). Myasthenia gravis is a type of disorder in which the muscles get weak and tire easily as a result of poor conduction (transmission) of nerve impulses. Guillain-Barre syndrome is a rare disorder that causes weakness in the arms and legs because of nerve damage.

WHAT ELSE ARE LYMPHOCYTES CALLED?

Lymphocytes are also known as lymph cells, lymphocysts, and lympholeukocytes.

WHAT IS THE ORIGIN OF THE TERM, LYMPHOCYTE?

Lymphocyte comes from the Latin word "lympha" meaning "spring water," and the Greek word "kytos" meaning "cell." Put the two words together and you have "spring water cell."