The body’s defense system relies on specialized proteins known as antibodies, or immunoglobulins, produced by plasma cells in response to foreign invaders. Immunoglobulin G (IgG) is the most abundant of these proteins, making up roughly 75% of the total antibodies found in human blood circulation. Testing the concentration of this protein is a common way to assess the status and history of a person’s humoral immune response. Understanding normal IgG levels and what deviations suggest is fundamental to diagnosing various immune-related conditions.
The Role of Immunoglobulin G in Immunity
IgG is the primary antibody responsible for the body’s long-term immune memory, functioning as the defender during a secondary immune response. Once the immune system has encountered a pathogen, plasma cells quickly generate large amounts of specific IgG antibodies upon subsequent exposure, allowing for swift neutralization of the threat. These antibodies protect the body through several mechanisms, including opsonization, where the IgG coats a pathogen to flag it for destruction by phagocytic immune cells.
The IgG molecule can also neutralize toxins and viruses by binding directly to them, preventing them from entering host cells. IgG is unique among immunoglobulins because it is the only class that can cross the human placenta. This maternal IgG transfer provides the fetus and newborn with passive immunity during the first several months of life while their own immune system matures.
IgG is further divided into four subclasses—IgG1, IgG2, IgG3, and IgG4—each with specialized functions. IgG1 and IgG3 are effective against protein-based antigens, such as those found on viruses and toxins. Conversely, IgG2 is primarily responsible for the immune response against polysaccharide antigens, which are common components of the capsules surrounding certain bacteria like Streptococcus pneumoniae.
Defining Normal Reference Ranges
For adults, the typical reference range for total serum IgG concentration generally falls between 700 to 1600 milligrams per deciliter (mg/dL). The specific “normal” range is determined by each individual testing laboratory based on its equipment and population, meaning results must always be interpreted using the lab’s provided figures. Age significantly influences IgG levels, necessitating the use of distinct reference ranges for pediatric patients.
An infant’s IgG level at birth is high, often mirroring the mother’s concentration due to placental transfer. These maternal antibodies naturally degrade over the first few months, causing a temporary dip in the infant’s IgG concentration, known as physiologic hypogammaglobulinemia of infancy. After this low point, the child’s own immune system begins to produce increasing amounts of IgG, reaching adult concentrations in adolescence.
Understanding Elevated IgG Results
An elevated IgG level, termed hypergammaglobulinemia, most commonly indicates an active immune response to a persistent or chronic stimulus. This polyclonal increase means many different B cell clones are producing a variety of antibodies to fight multiple antigens. Chronic infections, such as HIV, hepatitis, or tuberculosis, frequently result in a sustained high IgG level as the body attempts to clear the pathogen.
Chronic inflammatory conditions and autoimmune diseases also drive polyclonal IgG elevation, as the immune system mistakenly targets the body’s own tissues. Systemic lupus erythematosus and rheumatoid arthritis are examples where ongoing immune activation leads to a persistent rise in IgG. A different type of elevation is a monoclonal gammopathy, where a single, abnormal clone of plasma cells produces a massive amount of one specific, non-functional IgG protein. This is a hallmark of certain cancers, such as Multiple Myeloma, and requires further evaluation to distinguish it from a broad polyclonal increase.
Understanding Depressed IgG Results
Low IgG levels, or hypogammaglobulinemia, significantly impair the body’s ability to defend against encapsulated bacteria and viruses, leading to increased susceptibility to recurrent infections. These deficiencies fall into two main categories: primary (genetic) and secondary (acquired) causes. Primary Immunodeficiencies are inherited disorders that affect the development or function of antibody-producing cells, such as Common Variable Immunodeficiency (CVID). CVID is characterized by low IgG and recurrent bacterial infections, often of the respiratory tract.
Another severe primary deficiency is X-linked agammaglobulinemia (XLA), where the body fails to produce almost all immunoglobulins due to a block in B cell development. Secondary causes of low IgG are more common and result from external factors, including the loss of protein through the body, such as in nephrotic syndrome or protein-losing enteropathy. Certain medications, particularly immunosuppressive drugs used in chemotherapy or transplant rejection, can suppress B cell function and lower IgG production. Hematologic malignancies like chronic lymphocytic leukemia also result in hypogammaglobulinemia by interfering with healthy plasma cell function in the bone marrow.