If your child has ever undergone immune system testing or allergy testing, you may have heard the term immunoglobulin, or the abbreviation for an immunoglobulin class, such as IgG or IgM. But what are immunoglobulins, and what are all those classes? How do they relate to the immune system, infections, and allergies?
Simply put, immunoglobulins are antibodies. Technically, there are two types of immunoglobulins, and only one of these types is the group of antibodies, but for general purposes, the two terms can be interchanged. The prefix “immuno” obviously points to the immune system, and “globulin” is just another word for a type of protein.
Immunoglobulins are produced by the white cells in the blood. Their primary role is to fight any invaders or toxins in the body, including bacteria, viruses, fungus, allergens, or any other form of toxin. Each antibody is Y-shaped, and the end of each Y contains anti-toxin or antibody that is specific to a certain type of invader. Each Y is specifically shaped so it fits together like a puzzle piece with a specific invader. So, for example, if your body contains an antibody to the flu virus and you are exposed to the flu, the antibody for flu will come and use its Y to bind to the invading flu virus in your body. It will, in effect, neutralize the invader and keep you from getting the flu.
Immunoglobulins are divided into five classes or isotypes in the human body. Each type fights a different class of invaders or toxins. The types are as follows:
- IgA (Immunoglobulin Alpha) – protects external parts of your body, including the nasal passages, ears, eyes, and digestive tract, from foreign substances.
- IgE (Immunoglobulin Epsilon) – found in the lungs, skin, and mucus membranes, these antibodies respond to allergens and trigger histamine to be produced, causing an allergic reaction.
- IgG (Immunoglobulin Gamma) – the most common type of antibody, these immunoglobulins fight bacteria, viruses, and other invaders.
- IgM (Immunoglobulin Mu) – these antibodies are the “first responders” who target infections, or call out other types of immunoglobulins to respond to toxins.
- IgD (Immunoglobulin Delta) – we don’t understand fully what these immunoglobulins do yet.
The immunoglobulin G class can be further divided into four subtypes, while the immunoglobulin A class can be divided into two subtypes.
Immune Deficiencies and Titers
If a child gets frequent bacterial infections, a doctor may suspect she has an immune deficiency. Typically, children with immune deficiencies have low levels of IgG (or one or more specific IgG subclasses) in their blood, though they may also have low levels of IgA or IgM. In some cases, the immunoglobulins are present but do not work well. There are more than 250 different types of immune deficiencies, and each presents with a different pattern. While low levels of IgG are the most common presentation of immune deficiency, there are many other possibilities. For example, cellular immunodeficiencies, such as DiGeorge syndrome, affect the function of T-cells, which are specialized forms of white blood cells.
One way to identify an immune deficiency is to test for IgG levels. In addition, doctors may test for specific IgG titers, or concentrations of antibodies in the blood. For example, after being vaccinated for measles, a doctor may test a child for antibodies to the measles virus to see if the child is able to make antibodies. If a child cannot make antibodies to any vaccines or known invaders, he is likely to have an immune deficiency. If a child makes antibodies to some vaccines or known invaders but not others, he may have a specific antibody deficiency to a class of invaders.
Children who do not have enough IgG, or who have other types of immune deficiencies, may receive Immunoglobulin therapy. This treatment pools the immunoglobulins from thousands of blood donors to replace missing or ineffectual immunoglobulins in a child. Typically, it is called IVIg (Intravenous Immunoglobulin) or SCIg (Sub-cutaneous Immunoglobulin), depending on the route of administration.
In autoimmune diseases, the body attacks itself, and children may have increased overall antibodies, especially IgA. Measuring immunoglobulin levels can help doctors to diagnose and monitor treatment of autoimmune diseases, such as rheumatoid arthritis, lupus, and multiple sclerosis. Interestingly, in some severe cases of autoimmune disease such as neuropathies, immunoglobulin therapy may also be used.
Children with severe allergies to food or other substances, including pollens, may also receive immunological testing. These children may have overall increased levels of Immunoglobulin E (IgE), or they may just have increased levels of IgE specific to a particular allergen. For example, a child with a peanut allergy will have extremely high levels of IgE that is specific to peanuts.
An allergist or immunologist can test various specific levels of IgE to a wide range of substances using a RAST test. High levels to a specific potential allergen indicate a child may have an allergy to that substance. While this test is not 100% accurate in predicting allergies, it can be a starting point for diagnosis, along with skin testing and food trials.
Still a Young Science
Every day, scientists are learning more about immunoglobulins and the function of the immune system. There is still much we do not understand. But knowing about immunoglobulins and their relationship to conditions and symptoms can really help diagnose and treat a child.