Antibodies are specialized Y-shaped proteins made by the immune system. They help fight disease by detecting viruses, bacteria, and other pathogens (disease-causing microorganisms) and working to destroy them. Harmful infectious organisms are identified as invaders due to their antigens, which are distinct molecules on their surface. Each antibody produced by your immune system binds to a specific antigen—with a fitted molecular shape—and then either destroys the pathogen or tags it so other immune cells will recognize it.
Immunoglobulin Isotypes
Antibodies are also known as immunoglobulins (Ig). Immuno describes immunity and globulin describes protein. They are produced by B cells, a specific type of white blood cell (WBC) that originates in the bone marrow.
While there are only five main types of antibodies, each antibody can have a different binding site that matches a specific antigen. In fact, your body can produce an infinite number of binding sites to bind to antigens.
Immunoglobulin G (IgG)
Immunoglobulin G (IgG) accounts for around 75% of all antibodies in the human body. Depending on the antigen, IgG can either tag a pathogen so other immune cells and proteins will recognize it, or it can activate the complement system to directly destroy the microorganism.
Immunoglobulin A (IgA)
Immunoglobulin A (IgA) is primarily found in mucosal tissues, such as those in the mouth, vagina, and intestines, as well as in saliva, tears, and breast milk. It accounts for 15% of all antibodies in the human body and is produced by B cells and secreted from the lamina propria, a thin layer within mucosal tissues.
IgA is one of the body’s first-line defenses against infection. It binds to pathogens to tag them for destruction and prevents them from sticking to the epithelium, which lines the body’s tissues.
IgA is also associated with hypersensitive reactions in people with celiac disease and several other autoimmune disorders.
Immunoglobulin M (IgM)
Immunoglobulin M (IgM) is also one of the first antibodies recruited by the immune system to fight infection. IgM populations rise very quickly when the body is first confronted with an infectious organism, and then they plummet as IgG antibodies take over. IgM is also produced by B cells and, when bound to a pathogen, will spur other antibodies and immune cells into action.
In addition to activating the immune response, a subset of IgM helps B cells “remember” a pathogen after it has been destroyed. If you were to become re-exposed to the pathogen later, your immune system should respond more quickly due to your memory B cells.
Immunoglobulin E (IgE)
Immunoglobulin E (IgE) is the antibody responsible for the allergic response that is mostly found in the lungs, skin, and mucosal membranes. IgE is produced by B cells secreted by lymph nodes or other lymphoid tissues situated near the site of the allergen (a harmless substance that induces an allergic response).
When IgE binds to an allergen, it triggers a cascade of events. Basophils and mast cells, which are subtypes of WBCs, degranulate (break open) and release histamine, an inflammatory compound, into the bloodstream. It is histamine that is responsible for many of the most common symptoms of allergy.
Immunoglobulin D (IgD)
Immunoglobulin D (IgD) is important in the early stages of the immune response. Unlike other antibodies, it does not actively circulate but instead binds to B cells to instigate the immune response. As a signaling antibody, IgD helps incite the release of front-line IgM to fight disease and infection.
IgD accounts for only around 0.25% of antibodies in the human body. Despite its vital role in “kick-starting” the immune response, IgD is arguably the least understood antibody, with little known about how it might participate in other parts of the immune system.
Antibody Testing
Because immunoglobulins are matched to a specific pathogen, they can be used to diagnose some diseases based on their unique structure. Antibody tests are used to detect disease-specific antibodies in a blood sample.
Antibody tests are available to diagnose (or help diagnose) a wide variety of infectious and autoimmune diseases, including:
Celiac disease (CD) COVID-19 Coxsackievirus Cytomegalovirus (CMV) Diphtheria Epstein-Barr virus (EBV) H. pylori HIV Influenza Lyme disease Mumps Mycoplasma pneumonia Pertussis (whooping cough) Polio Rubella (German measles) Syphilis Tetanus Toxoplasmosis Varicella-zoster virus Viral hepatitis West Nile virus
Antibody tests do not detect the actual pathogens that cause an infection—they detect the antibodies that are produced in response to the infection. A positive result means “yes,” the test has detected the antibody or antigen. A negative result means “no,” while borderline results are considered inconclusive.
Depending on the disease, it may take time for enough antibodies to be produced to reach detectable levels. If it’s done too soon, during the early window period, the test may deliver a false negative result.
And some disorders are associated with a deficiency, not from a specific IgG. For example, primary immunodeficiency disease (PID) is diagnosed with a deficiency of total IgG.
Sometimes, immunoglobulin levels can be used to characterize the stage of an infection. Because IgM levels usually increase before the IgG response kicks in, a disease-specific IgM and IgG test can help determine whether an infection has occurred recently. For example, herpes simplex is an infection for which IgM and IgG tests can help determine the timing of the infection.
In people with allergies, IgE tests can be used to confirm that an allergic response has occurred.
A Word From Verywell
When taking an antibody test, it is important to remember that antibodies are produced in response to a disease or infection; they are not the disease or infection. There are, in fact, some antigen tests that can detect an actual pathogen by its signature antigen.
Some diseases can be diagnosed with either an antibody or antigen test. In other cases, only an antibody or antigen test is available.
Your healthcare provider or clinic can tell you the window period for your infection so you can get accurate results.
Immunoglobin A (IgA) is found in mucosal tissue and is the front line defense against infection. IgA binds to pathogens to tag them for destruction from other antibodies. IgA is also associated with celiac disease and other autoimmune disorders. IgD binds to B cells to kick start the immune response. IgG works in two ways: It binds to a pathogen to alert other immune cells to attack it or promotes activation of the complement system to destroy the invader. In people with autoimmune diseases, certain IgGs can trigger a symptom flare. IgE is the antibody responsible for allergic reactions. IgE binds to an allergen, triggering a release of histamine, which causes allergy symptoms. IgE also helps to fight parasitic infections. IgM is one of the first antibodies called in to fight infection. When it binds to a pathogen, it prompts the release of other antibodies, like IgG. IgM also serves as the memory bank of the immune system, recalling pathogens that have already been destroyed. IgM helps provide immunity to illnesses you have already had or have been vaccinated against.
