Antibody Tests, Immunoglobulins

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Antibody Tests, Immunoglobulins

Definition
Purpose
Precautions
Description
Preparation
Aftercare
Risks
Results

Definition

Antibodies, also called immunoglobulins, are proteins produced by the body’s immune system that are responsible for fighting off various invaders, such as viruses, bacteria, toxins, and mold spores. They work to clear the body of potentially threatening infections or substances.

The body’s immune system is made up of lymphoid organs, including lymph nodes, the bone marrow (located within the center of long bones) and the thymus (located in the chest). These lymphoid organs produce lymphocytes, including T cells and B cells. These lymphocytes circulate within the bloodstream, within the lymph system, and are also positioned in clumps within organs and on mucosal surfaces of the body. When a B cell encounters a foreign invader, it recognized it as foreign by virtue of a chemical identifier on its surface (called an antigen). Once the B cell recognizes an antigen, the B cell gives rise to a large number of plasma cells. These plasma cells are capable of producing antibodies.

Antibodies are made up of units called “chains.” All antibodies are composed of two larger chains (called heavy chains) and two smaller chains (called light chains). The tip of the antibody is referred to as the hypervariable region. This hypervariable region is responsible for unique chemical properties possessed by each antibody that allow a specific antibody to “recognize” and match up to a particular antigen. The combination of an antibody with a specific antigen, creates an antibody-antigen complex, marking the invader as foreign and in need of inactivation or destruction by other immune cells in the body.

The first time an antigen is encountered by the immune system, the body’s response is slow. Time is required in order to activate the machinery necessary to produce the very specific type of antibody necessary to combat that antigen; however, if that particular antigen is encountered in the future, the needed machinery is already available, and antibody production in response to a “familiar” antigen is quite rapid.

Antibodies are divided into five different specific classes of immunoglobulins, termed IgA, IgG, IgM, IgE, and IgD. Each of these classes of immunoglobulins has different characteristics, including overall percentage of immunoglobulins, location, timing of action, and type of antigen to which it attaches:

About 80% of all circulating antibodies are IgG. IgG is found in blood and tissue fluids. It coats invading particles, marking them so that they can more easily and rapidly be taken up by other types of immune cells. IgG is the predominant antibody cell in the later or secondary phase of immune response.
IgM makes up about 13% of all antibodies. IgM is primarily found in the blood. It functions to kill bacteria, and is found in the earlier phases of immune response to bacterial invasion of the bloodstream (bacteremia).
IgA makes up about 6% of the body’s total antibodies. IgA is found in large quantities in a variety of bodily fluids, such as breast milk, tears, saliva, and on the surface mucosal lining of the respiratory and digestive tracts. In these locations, IgA is poised to protect these areas that serve as entrances to the body.
IgE is the least prevalent antibody, composing about 0.002% of the body’s total antibodies. IgE is found bound to immune cells called basophils and mast cells. It is involved in fighting parasites, and is also the predominant antibody seen in allergic reactions.
Only about 1% of the body’s antibodies are IgD. IgD primarily stays attached to B cells, and helps mediate the B cells’ early response to antigen exposure. IgD antibodies are particularly active in newborn babies.

One of the important attributes of a healthy, well-functioning immune system rests on its ability to distinguish between “self” and “other.” This means that it’s important that the antibodies don’t mistakenly identify parts of the body itself as foreign invaders. When this does happen, the body’s immune system attacks the body, damaging and destroying it. Conditions in which this occurs are referred to as autoimmune disorders. One example of an autoimmune disorder is the condition called rheumatoid arthritis or RA. In RA, the lining of the joints is mis-recognized by the immune system as foreign, resulting in the immune system creating specific antibodies that repeatedly attack, damage, and destroy the joints’ lining, resulting in the severe symptoms that accompany this disease.

Another way that the immune system can accidentally work against the body involves the reaction known as allergy or hypersensitivity reactions. In this situation, the immune system reacts overly strongly to a commonly-encountered substance, such as pollen, animal dander, a food ingredient, or an antibiotic medication. While most people’s immune systems do not respond to these substances as antigens, an allergic individual’s immune system identifies some aspect of the substance as an antigen, triggering an immune reaction. As a result of the ensuing immune response, the individual experiences symptoms of allergy, which are secondary to the immune system’s overly-exuberant response to a substance that is usually ignored by most people’s immune systems. Allergic responses can vary from mild reactions to overwhelming, life-threatening (anaphylactic) responses.

Strong activation of the immune system to specific chemical markers on transplanted organs is the phenomenon responsible for organ rejection. In this instance, the individual’s immune cells identify the transplanted organ’s cells as foreign invaders, and specific antibodies that match the organ’s antigens are produced. The organ is attacked by the immune system, and damaged, interfering with the organ’s functioning or even destroying it. This same phenomenon is responsible for a transfusion reaction; the individual’s immune system reacts to the presence of a foreign antigen within the transfused blood, kicking off an immune reaction. The blood cells are attacked by the body’s immune cells, and a transfusion reaction ensues.

An understanding of the antibody response is harnessed and used to advantage in the preparation of vaccines or immunizations. In this instance, the vaccine is given in order to “introduce” the body to a particular viral invader that it may encounter in the future. This is done by inactivating the virus (that is, making it unable to actually cause illness). The inactivated virus still has its identifying surface antigen present, allowing the immune system to become acquainted with it. After this introduction, if the individual is actually exposed to that virus, the immune response will be rapid, which will either prevent any illness that occurs due to that virus, or result in a less-severe, shorter course of illness.

Purpose

Immunoglobulin or antibody tests may provide quantitative or qualitative information. Quantitative testing reveals the levels of a particular antibody. Qualitative testing is done to demonstrate the presence or absence of a specific type of antibody.

Immunoglobulin or antibody tests are performed in order to:

verify that an individual has been exposed to a particular microbial agent or substance (IgG or IgM testing for infectious agents, IgA testing for allergic exposures);
check to see whether an individual is immune to a particular microbial agent (IgG or IgM testing);
diagnose and/or monitor an autoimmune disorder;
ascertain the reason for organ rejection or a transfusion reaction;
diagnose an allergy (IgE and/or IgA testing);
verify that you are immune to a particular disease (sometime used to make sure that an immunization was effective);
monitor treatment for the bacteria that causes stomach ulcers (Helicobacter pylori);
monitor treatment for cancers that affect the functioning of the bone marrow;
diagnose multiple myeloma or macroglobulinemia (types of cancer that affect immune cells); and
diagnose and/or monitor the course of an infection (usually IgG and IgM testing). This may require two samples, one during the height of the illness (called the acute sample) and one some weeks later (called the convalescent sample). IgM is usually present in the case of a recent infection; IgG is usually present in the event of an infection that occurred at some point in the past.

Precautions

A number of situations may skew the test results, and should be taken into account when planning an antibody test. These situations include:

the use of certain medicines, such as birth control pills, antiseizure medications (including phenytoin), corticosteroids, methotrexate, asparaginase, amino-phenazone, phenylbutazone, and hydralazine;
recent cancer treatment (radiation and/or chemotherapy);
having received a blood transfusion within the previous six months;
recent (within the previous six months) immunizations, especially those requiring repeat booster doses;
recent use of alcohol or illegal drugs; and
recent radioactive scan (within the three days previous to immunoglobulin testing).

Description

This test requires blood to be drawn from a vein (usually one in the forearm), usually by a nurse or phlebotomist (an individual who has been trained to draw blood). A tourniquet is applied to the arm above the area where the needle stick will be performed. The site of the needle stick is cleaned with antiseptic, and the needle is inserted. The blood is collected in vacuum tubes. After collection, the needle is withdrawn, and pressure is kept on the blood draw site to stop any bleeding and decrease bruising. A bandage is then applied.

Preparation

There are no restrictions on diet or physical activity, either before or after the blood test.

Aftercare

As with any blood tests, discomfort, bruising, and/or a very small amount of bleeding is common at the puncture site. Immediately after the needle is withdrawn, it is helpful to put pressure on the puncture site until the bleeding has stopped. This decreases the chance of significant bruising. Warm packs may relieve minor discomfort. Some individuals may feel briefly woozy after a blood test, and they should be encouraged to lie down and rest until they feel better.

KEY TERMS

Antibody— A protein that the body produces in response to exposure to a foreign invader such as a virus, bacteria, fungus, or allergen.

Antigen— The protein marker that prompts the body’s immune system to produce antibodies.

Autoimmune disorder— A condition in which the body produces antibodies that serve to attack organs or tissues of the body itself.

Immune system— The collection of organs, tissues, and cells that serve to protect the body against foreign invaders, such as bacteria, viruses, and fungi.

Lymphocyte— A white blood cell; part of the immune system responsible for the production of antibodies.

Plasma cell— The specific type of white blood cell that produces antibodies.

Risks

Basic blood tests, such as immunoglobulin or antibody testing, do not carry any significant risks, other than slight bruising and the chance of brief dizziness.

Results

Antibody tests are performed by mixing a sample of the patient’s blood with a sample containing a known, identified antigen. If the patient’s blood contains antibody to that antigen, then the antibody will bind to the antigen, creating an antibody-antigen complex. This complex can be measured. Depending on the reason for testing, results may be reported very simply as “detected” or “not detected.” Alternatively, results may report on whether the amount of complex detected exceed a predetermined level, one which might reflect the individual’s immune status to the antigen-containing substance. In this case, the resulting laboratory report might read “immune” or “not-immune.” Lastly, the results might be reported as a concentration, in milligrams per deciliter (mg/dL) or grams per liter (g/L).

Normal results for antibody concentrations are as follows:

Ig: 85-385 mg/dL or 0.85-3085 g/L
IgG: 565-1765 mg/dL or 5.65-17.65 g/L
IgM: 55-375 mg/dL or 0.55-3.75 g/L
IgD: Less than 8 mg/dL or 5-30 micrograms per liter
IgE: 10-1421 micrograms per liter

High levels

High levels of IgA may indicate a monoclonal gammopathy, the presence of multiple myeloma, autoimmune disease(rheumatoid arthritis or systemic lupus erythematosus, for example), or liver disease (including cirrhosis of the liver or chronic hepatitis).

High levels of IgG may indicate the presence of a chronic infection (including AIDS), or multiple myeloma, chronic hepatitis, or multiple sclerosis.

High levels of IgD may indicate multiple myeloma.

High levels of IgE may indicate the presence of a parasitic infection, as well as an allergic response, asthma, atopic dermatitis, autoimmune disease, cancer or multiple myeloma.

Low levels

Abnormally low levels of IgA may occur in the presence of leukemia, nephritic syndrome, intestinal diseases, rare congenital immune deficiencies of IgA, or a rare genetic disease called ataxia-telangiectasia.

Abnormally low levels of IgG may occur in macroglobulinemia, leukemia, nephritic syndrome, and rare congenital immune deficiencies of IgG.

Abnormally low levels of IgM may occur in the presence of multiple myeloma, leukemia, and some genetic immune disorders.

Abnormally low levels of IgE may occur in the presence of ataxia telangiectasia.