A healthy result should fall into the range 41 - 129 mg/dL.

There are five classes of antibodies (immunoglobulins) that help the body fight infection: IgG, IgA, IgM, IgD, and IgE. Immunoglobulins make up the antibodies that fight infection. Most of the antibodies in the blood and other fluids that surround the body’s tissues and cells are of the IgG class. IgG is divided into four subclasses: IgG1, IgG2, IgG3, and IgG4. People are said to have an IgG subclass deficiency when they lack or have very low levels of one or two IgG subclasses, but have normal levels of other immunoglobulins.

Each IgG subclass plays a slightly different role in protecting the body against infection. Therefore, a person lacking a specific IgG subclass will be vulnerable to certain kinds of infections but not others. For example, the IgG1 and IgG3 subclasses contain antibodies against serious bacterial infections such as diphtheria and tetanus, as well as antibodies against viral proteins. In contrast, the IgG2 subclass contains antibodies against the polysaccharide coating of certain disease-producing bacteria that can cause ear and sinus infections, pneumonia, blood infections and meningitis.

Conditions of IgG Subclass Deficiency:

Recurring ear and sinus infections, sinusitis, bronchitis, and pneumonia are the most frequent illnesses in people with IgG subclass deficiencies. Some people have an increased frequency of infections by an early age; others will not experience infections until later.

Diagnosis of IgG Subclass Deficiency:

A person is considered to have a selective IgG subclass deficiency if blood levels of one or more of the IgG subclasses are below the normal range based on age, and the levels of other immunoglobulins (total IgG, IgA, and IgM) are normal or near normal.

As a person may have no or very low levels of one or more IgG subclasses while maintaining a normal level of total IgG, diagnosis requires measurement of IgG subclasses in addition to serum IgG, IgA, and IgM. IgG4 deficiency often occurs in combination with IgG2 deficiency.

It’s important to remember that the definition of “normal” IgG subclass concentrations varies over time, and from lab to lab. Normal values usually represent a small range below and above the average for a person’s age.

An additional subset of patients have normal levels of immunoglobulin and normal IgG subclasses, yet fail to develop protective antibody levels in response to infections with Streptococcus pneumoniae or vaccines against the bacteria. These people are thought to have a specific antibody deficiency and are usually grouped with patients with IgG subclass deficiency.

Measurement and interpretation of IgG Subclass levels:

IgG subclass deficiency is defined as a reduction in 1 or more subclasses of IgG (IgG1 and IgG2 in particular) by more than 2 SDs below the mean for age-matched controls. Deficiencies in IgG1 or IgG2 are more likely to cause low serum IgG levels because these are the major components. IgG1 levels stabilize by the age of 5 years; however, adult levels of IgG2, IgG3, and IgG4 may not be achieved until adolescence.16 Thus, the interpretation of subclass levels is additionally problematic because levels of each subclass increase at different rates during development. The significance of isolated IgG subclasses is controversial. Laboratories have different age-appropriate reference ranges; aliquots of the same serum may result in different levels from different laboratories.

15 Reduced levels of 1 or more subclasses may be found in healthy individuals, making precise clinical interpretation challenging. If performed, proper evaluation of IgG subclasses should include measurements on 2 separate occasions at least 1 month apart while the patient is free of infections.6 Determination of IgG subclasses may be useful in patients with selective IgA deficiency, in which IgG2 deficiency is found in approximately 10% of patients.15 However, quantitation of IgG subclasses provides insufficient information about antibody function; therefore, specific functional antibody response to proteins and polysaccharide antigens is important. Without clear deficits of functional antibody, IgG subclass deficiency is not likely to be clinically significant.

Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3099256/

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