Immunoglobulins, commonly known as antibodies, are specialized Y-shaped proteins produced by B cells and plasma cells, a type of white blood cell. They circulate throughout the body, including in blood plasma, to identify and neutralize foreign substances like bacteria, viruses, and toxins. These molecules are central to the body’s defense against infection and disease.
The Five Classes of Immunoglobulins
The immune system produces five main classes of immunoglobulins: IgG, IgA, IgM, IgE, and IgD. Each class has distinct structures and roles in protecting the body, with differences stemming from variations in their heavy chain protein sequences. This structural diversity allows each class to perform specific functions in different parts of the body or at various stages of an immune response.
Immunoglobulin G (IgG) is the most abundant class, making up approximately 70% to 75% of the total immunoglobulin pool in blood and tissue fluids. IgG antibodies effectively bind to bacteria and toxins, playing a significant role in long-term immunity against previously encountered pathogens. Uniquely, IgG is the only antibody class capable of crossing the placenta, providing protective antibodies from the mother to a developing fetus and newborn.
Immunoglobulin A (IgA) is predominantly found in mucosal secretions like saliva, tears, sweat, nasal fluids, and breast milk. While it exists as a single unit (monomer) in the bloodstream, IgA forms a double unit (dimer) in these secretions. Its primary function is to provide localized protection on mucosal surfaces, acting as a first line of defense by preventing microorganisms from attaching to and entering the body’s epithelial linings.
Immunoglobulin M (IgM) is the first antibody produced by B cells when the body encounters a new infection or antigen. This class is found as a large pentamer, a structure composed of five Y-shaped units, providing 10 antigen-binding sites. This multi-site binding capability makes IgM highly effective in the initial stages of an immune response, particularly in controlling bacterial invaders within the bloodstream.
Immunoglobulin E (IgE) is present at very low concentrations in the serum. Its levels can increase notably during allergic reactions and certain parasitic infections. IgE antibodies bind to specialized cells called mast cells and basophils. When these IgE-bound cells encounter an allergen, they release inflammatory substances like histamine, contributing to the symptoms associated with allergies.
Immunoglobulin D (IgD) is found on the surface of B cells, functioning as an antigen receptor. While its exact roles are still being investigated, IgD plays a part in the activation and differentiation of B cells. It also contributes to the induction of antibody production and is involved in the prevention of respiratory tract infections.
How Immunoglobulins Protect the Body
Immunoglobulins protect the body through several distinct mechanisms, each designed to neutralize or eliminate foreign invaders. These actions are highly specific, targeting the antigens that triggered their production. Antibodies do not directly destroy pathogens; instead, they tag them or interfere with their functions, paving the way for other immune components to complete the elimination process.
One mechanism is neutralization, where antibodies bind directly to the surface of pathogens or to toxins they produce. This binding blocks the pathogen’s ability to attach to host cells, similar to placing a cover over a keyhole. This prevents attachment or entry, effectively neutralizing the pathogen’s capacity to cause infection or harm.
Another protective action is opsonization, where antibodies “tag” invaders for destruction. An antibody coats the surface of a pathogen, making it more recognizable for phagocytic cells like macrophages and neutrophils. These immune cells have specific receptors that bind to the antibody-coated pathogen, enhancing the efficiency of its engulfment and subsequent breakdown.
Immunoglobulins also contribute to complement activation, initiating a cascade of proteins that eliminate pathogens. When certain antibody classes, particularly IgM and IgG, bind to antigens on a pathogen’s surface, they trigger the complement system. This activation leads to events that can directly rupture the pathogen’s membrane or further enhance opsonization and inflammation.
Measuring Immunoglobulin Levels
Healthcare providers may order an immunoglobulin test to assess an individual’s immune system health. This blood test measures the levels of the three major immunoglobulin classes: IgG, IgA, and IgM. A doctor might recommend this test when investigating symptoms such as recurrent infections, chronic inflammation, or suspected autoimmune conditions, to gain insight into the body’s antibody production.
Abnormal immunoglobulin levels can indicate various underlying health issues. If levels are lower than expected, hypogammaglobulinemia may be present, suggesting an immunodeficiency. This deficiency can lead to increased susceptibility to infections, particularly in the respiratory and digestive tracts.
Conversely, higher-than-normal immunoglobulin levels can also signal an immune system imbalance. Elevated levels might be observed during ongoing infections, chronic inflammatory states, certain autoimmune diseases like rheumatoid arthritis or lupus, or some types of blood cancers. Even with high levels, antibodies may not function correctly, potentially leaving the individual vulnerable to frequent infections. An immunoglobulin test alone does not provide a definitive diagnosis; further diagnostic evaluations are often necessary to determine the precise cause of abnormal results.
Immunoglobulins as a Medical Treatment
Immunoglobulins are not only diagnostic markers but also serve as a medical treatment for various conditions. Immunoglobulin replacement therapy, administered intravenously (IVIG) or subcutaneously (SCIG), provides patients with concentrated antibodies derived from the plasma of thousands of healthy human donors. These preparations consist primarily of IgG, offering a broad spectrum of protective antibodies.
One primary application of this therapy is to replace missing or deficient antibodies in individuals with primary immunodeficiencies. These are genetic or acquired conditions where the body’s immune system cannot produce sufficient functional antibodies. By infusing donated immunoglobulins, the therapy provides necessary protective antibodies, reducing the patient’s risk of severe and recurrent infections.
Beyond replacement, immunoglobulin therapy also modulates the immune system in certain autoimmune or inflammatory disorders. In these conditions, the immune system mistakenly targets and damages the body’s own tissues. While exact mechanisms are complex, administered immunoglobulins help regulate this overactive immune response. This therapy offers temporary protection and requires regular, lifelong administration to maintain therapeutic levels.