Immune globulin (IG), also known as antibody, is a protein produced by the body’s adaptive immune system to identify and neutralize foreign invaders. These Y-shaped molecules are manufactured by specialized white blood cells called plasma cells, which are the mature form of B lymphocytes. When the immune system encounters a foreign substance, known as an antigen—such as a protein on a virus or bacterium—B cells become activated and differentiate into plasma cells. The plasma cells then secrete massive amounts of these specific antibodies into the bloodstream and other body fluids.
The primary function of an immune globulin is to act as a highly specific search-and-destroy tag, binding precisely to the antigen that triggered its production, much like a lock and key. This specific binding action is what allows the body to target and eliminate a nearly infinite variety of pathogens it may encounter over a lifetime. Immune globulins are the core components of what is called humoral immunity, providing a sophisticated, long-lasting defense against infectious agents.
The Five Classes of Immune Globulin
The human body produces five distinct classes of immune globulin, each playing a unique role in the overall defense strategy: Immunoglobulin G (IgG), IgA, IgM, IgE, and IgD. These classes differ in their structure, location in the body, and their primary mechanisms of action.
- Immunoglobulin G (IgG): This is the most abundant antibody, representing about 75% to 80% of all antibodies in the blood and tissue fluids. It is the only class capable of crossing the placenta, providing passive immunity to a developing fetus and protecting the newborn in its first months of life. Structurally, IgG typically exists as a single Y-shaped unit, or monomer, making it small enough to readily leave the bloodstream and enter tissues to fight infection.
- Immunoglobulin A (IgA): IgA is the main antibody found in mucosal secretions, including saliva, tears, breast milk, and the linings of the respiratory and digestive tracts. It forms a dimer structure, meaning two Y-shaped units are joined together, which helps it survive in the harsh environments of the mucosal surfaces. IgA functions mainly to prevent pathogens from adhering to and colonizing these vulnerable surfaces.
- Immunoglobulin M (IgM): This is the largest antibody, typically existing as a pentamer—five Y-shaped units joined in a star-like cluster with ten binding sites. This massive structure makes it a very effective first responder, as it is the first class of antibody produced during a primary immune response to a new infection. IgM is primarily found in the blood and lymph fluid, where its large size allows it to efficiently clump, or agglutinate, invading microbes.
- Immunoglobulin E (IgE): IgE is present in relatively low concentrations but is associated with allergic reactions and defense against parasites. IgE binds to mast cells and basophils, and when it encounters an allergen or parasite antigen, it triggers the release of histamine and other chemicals that cause the symptoms of allergy.
- Immunoglobulin D (IgD): IgD is found in small amounts in the blood and, along with IgM, serves mainly as a receptor on the surface of B cells, helping to initiate the B cell’s activation and differentiation.
How Immune Globulins Protect the Body
Immune globulins protect the body through several distinct and coordinated mechanisms once they have successfully bound to an antigen.
Neutralization
Neutralization is a direct action where the antibody physically blocks a pathogen from causing harm. An antibody can bind to the surface proteins of a virus, preventing the virus from attaching to and entering a host cell. Similarly, antibodies can neutralize bacterial toxins by binding to them and rendering them incapable of damaging the body’s tissues.
Opsonization
Opsonization is a process that tags the invader for consumption by immune cells. Once an antibody coats a bacterium, the constant region of the Y-shape (the Fc region) acts as a signal flag. Phagocytic cells, such as macrophages and neutrophils, have receptors that recognize this flagged region and are prompted to engulf and destroy the antibody-coated pathogen. This coating significantly increases the efficiency of pathogen clearance.
Complement Activation
Immune globulins also have the capacity to initiate the complement activation cascade. The complement system is a complex group of proteins that circulate in the blood and can be triggered by the binding of certain antibodies, particularly IgM and IgG, to a pathogen’s surface. This activation leads to a cascade of reactions that ultimately result in the formation of a membrane attack complex, which punctures the pathogen’s cell membrane and causes it to lyse, or burst.
Therapeutic Applications of Immune Globulin
The concentrated use of immune globulin, often referred to as IG therapy, represents a powerful medical intervention for a range of complex conditions. This treatment involves administering purified antibodies derived from thousands of healthy human plasma donors, providing a broad spectrum of protective antibodies. The applications for this therapy broadly fall into two main categories: replacement therapy and immunomodulation.
Replacement Therapy
Replacement therapy is used primarily for patients with Primary Immune Deficiencies (PIDs). These are genetic disorders where the body is unable to produce sufficient amounts of its own functional antibodies. These patients have severely compromised immune systems and are highly susceptible to recurrent, severe bacterial infections. Administering therapeutic IG products provides the missing antibodies, effectively restoring the patient’s humoral immunity and significantly reducing the frequency and severity of infections.
Immunomodulation
The second major use is immunomodulation, where large doses of IG are used to regulate an overactive or misdirected immune system in autoimmune or inflammatory diseases. In conditions like Kawasaki disease or Guillain-Barré syndrome, the immune system mistakenly attacks the body’s own tissues. The high concentration of therapeutic IG is thought to work by saturating receptors on immune cells, interfering with the signaling of autoantibodies, and dampening the overall inflammatory response. Common preparations for this therapy include Intravenous Immunoglobulin (IVIG) and Subcutaneous Immunoglobulin (SCIG).
Sourcing and Administration
Therapeutic immune globulin products are a biological medicine sourced exclusively from pooled human plasma. To ensure a wide range of protective antibodies, plasma is collected from tens of thousands of volunteer donors. This pooled plasma undergoes an extensive and highly regulated fractionation and purification process to isolate and concentrate the IgG antibodies, which are the most therapeutically relevant.
Safety is paramount, and the purification process includes multiple steps designed to inactivate or remove potential viruses and other pathogens. Rigorous screening of all individual plasma donations is performed before pooling, and the subsequent manufacturing steps provide a high degree of assurance regarding the product’s safety profile.
The two main methods of delivering therapeutic IG are intravenous and subcutaneous. Intravenous Immunoglobulin (IVIG) is typically infused in a clinic or hospital setting over several hours, usually every three to four weeks, allowing for rapid delivery of a large dose. Subcutaneous Immunoglobulin (SCIG) is injected directly under the skin, often using a small pump, and is typically administered more frequently. SCIG allows patients to safely self-administer the treatment at home, offering greater flexibility and a more consistent level of antibodies in the bloodstream.