What are antibodies?
Antibodies, also known as immunoglobulins, are proteins which are found in blood or other bodily fluids, and are used by the immune system to identify and neutralise foreign objects, such as bacteria and viruses. This response by the body is called a 'protective response' as the body attempts to destroy the invader and maintain health.
Antibodies don't work on their own; there are many systems that work together to protect the body, including white blood cells and complement. Antibodies are specific to each type of foreign substance. For example, antibodies made in response to a tuberculosis infection will only react to tuberculosis bacteria. Antibodies also work in allergic reactions. In some people antibodies are made against the body's own tissues, which is called an autoimmune disease.
You can be born with an immune system which makes low or absent levels of antibodies, your system may make low levels because of certain diseases such as cancer, or your immune system may become abnormal some time after birth. In such cases, you may have a higher chance of developing repeated infections.
Types of immunoglobulins
There are five distinct types of antibodies, called IgA, IgG, IgM, IgE and IgD. The Ig stands for Immunoglobulin, which means that each antibody molecule is made up of a special protein called a 'globulin', and this globulin is associated with the immune system ('immune-'). Each of these five antibodies has a specific role in the immune system.
IgA is found mainly in mucosal areas (the surfaces exposed to the outside world through orifices such as the mouth, respiratory tract, and vagina) and account for 10-15% of all antibodies. IgA acts as the first line of defense against molecules in these areas, before they enter the bloodstream.
IgM is the largest of the antibodies and is found circulating in blood (about 5-10% of all antibodies in the body). They are produced as the first line of defence against a foreign molecule when it reaches the bloodstream. It seeks out antigens (proteins that cause the immune system to react) and can mop up many at a time. It is the first antibody present after birth, and protects the body while the other antibodies are developing.
IgG is found in all body fluids. It is the smallest but most common antibody (75-80%) of all the antibodies in the body. They are also the only type of antibody that can cross the placenta in a pregnant woman to help protect her baby. IgG is the most important antibody in the immune system's defence. It is produced in the bloodstream after the first-line IgM has started to mop up the invader.
In fact, once the body has established that the invader is a real threat, the immune system stops producing IgM and switches to IgG production. With the onset of infection and the development of IgG, a memory cell remembers the specific IgG that was created, so that even after the disease has been successfully suppressed, IgG remains in the body to ensure that the same invader is taken care of when it enters the body again. This is exactly what happens in the body when you receive an immunisation shot.
IgE is the found in the lungs, skin and mucous membranes. They are most important antibody in allergy of all types: hayfever, asthma, skin reaction such as hives and angiodema (tissue swelling), and anaphylactic (life-threatening) reactions. The only other role of IgE apart from the allergic reaction is for parasites and intestinal worms. It is not produced against viruses, bacteria and other disease-causing micro-organisms. However, about one-third of allergies are delayed. In this case the mechanism is not usually due to IgE but the other components of the immune system, e.g., the white blood cells.
IgD is an antibody of which its role is less defined. It is usually associated with helping other immune functions, such as 'switching' from one class of antibody to another.
How does one test for antibodies?
A blood test is done to measure the level of specific immunoglobulins in the blood. The level of each type of antibody can give your doctor an indication of the cause of a medical problem you may have, for example:
- autoimmune disease such as rheumatoid arthritis
- liver disease
- kidney damage
- parasite infestation
- certain types of cancer
- recurring infection due to low level of antibodies
Injection with antibodies (intravenous immunoglobulin, IVIG) is mainly used as treatment in 3 major categories:
- immune deficiencies
- inflammatory and autoimmune diseases such as rheumatoid arthritis and multiple sclerosis
- acute infections such as Kawasaki's Disease and paediatric HIV infection
IVIG is given to maintain adequate antibody levels to prevent infections and induces a short term form of immunity called passive immunity.
In conclusion, it is clear that the immune system has a vital role in defending the body against foreign proteins or diseases, but that in some individuals, a specific part of this protective mechanism may go awry. In such a case, doctors may test one or more specific areas to evaluate whether that part of the immune system is working adequately or not. For example, your specific antibody for German measles may be tested to see if you are protected from the disease, or people with recurrent boils or infections may have their immune system assessed to see if there is a specific antibody deficiency. Your immune system may be boosted by immunisation, or assisted in a number of situations where a specific antibody is low or absent.