There are five types of antibodies in the human body, each with its own function in protecting against infection and disease. Here is a look at the types of antibodies, their location in the body, and their functions.
Antibodies
Antibodies (Ab) or immunoglobulins (Ig) are large Y-shaped protein molecules that bind to unique markers called antigens that occur on pathogens (bacteria, viruses, etc.) and other foreign molecules. Antibodies function either as soluble molecules circulating in body fluids or as receptors attached to immune cells. In both cases they bind antigens, which marks pathogens for destruction or triggers immune cell responses.
In humans and other placental mammals, each antibody consists of four polypeptide chains: two identical heavy chains and two identical light chains. Disulfide bonds connect the chains. The region at the top of the “Y” has variable amino acids that form the antigen binding site. The different conformations arising from the various amino acid sequences allow antibodies to bind to an immense variety of antigens.
The 5 Types of Antibodies
Humans have five types of antibodies: immunoglobulin A (IgA), immunoglobulin D (IgD), immunoglobulin E (IgE), immunoglobulin G (IgG), immunoglobulin M (IgM). For biochemistry students learning the names, it helps to rearrange the order of the letters to form a mnemonic word: GAMED. These five classes are more formally known as isotypes. Some of the isotypes contain subclasses, which are indicated with numbers (e.g., IgA1, IgA2). Isotypes differ from one another by the sequence of their heavy chains, which are identified by the Greek letters alpha (α) for IgA, delta (δ) for IgD, epsilon (ε) for IgE, gamma (γ) for IgG, and mu (μ) for IgM.
| IgA | IgD | IgE | IgG | IgM | |
| Heavy chains | α | δ | ε | γ | μ |
| Number of antigen binding sites | 4 | 2 | 2 | 2 | 10 |
| Molecular weight (Da) | 385,000 | 180,000 | 200,000 | 150,000 | 900,000 |
| Percentage of total antibodies in serum | 13% | 1% | 0.02% | 80% | 6% |
| Crosses placenta? | no | no | no | yes | no |
| Fixes complement? | no | no | no | yes | yes |
| Functions | Tags targets for destruction. Secreted into mucous, saliva, colostrum, and tears. | B cell receptor that stimulates the release of IgM. | Binds to mast cells and basophils. Involved in allergies and antiparasitic response. | Binds to phagocytes. Most abundant antibody in serum. Main antibody involved in secondary responses. | Fixes complement and is the main antibody in primary responses. |
Immunoglobulin A (IgA)
Mucosal tissues, such as the mouth, intestines, and vagina, produce IgA. It also occurs in breast milk, tears, and saliva. IgA accounts for about 13% of the body’s antibodies. This antibody acts as the body’s first line of defense against infection. It tags antigens so that they are destroyed before they stick to the epithelial lining of body cavities.
IgA is associated with hypersensitivity reactions, including some autoimmune disorders and celiac disease.
This antibody occurs as a monomer in blood, but forms dimers in mucous. This is the antibody that helps protect breast-fed babies from infection.
Immunoglobulin D (IgD)
IgD is an antibody that acts in the early immune response to a pathogen. It isn’t actively circulating in the body. Instead, B cells produce it as a signaling antibody that stimulates the release of IgM antibodies.
Immunoglobulin E (IgE)
The lymph nodes and other lymphatic tissue secrete IgE. It is responsible for most allergic reactions. When IgE binds to an allergen, it causes basophils and mast cells to release histamine. Histamine causes inflammation and other allergy symptoms. But, IgE isn’t all bad. It protects the body from parasitic infections, such as from helminths (worms).
Immunoglobulin G (IgG)
IgG is the most abundant antibody in blood plasma, accounting for about 75–80% of serum antibodies. It either tags an antigen for removal (opsonization) or activates immune mechanisms such as complement or antibody-dependent cellular cytotoxicity.
While IgE is best-known for causing allergic reactions, IgG sometimes causes an undesirable response associated with autoimmune diseases. In these situations, IgG mis-identifies the body’s own tissues as pathogens.
IgG is the only antibody class that crosses the placenta, allowing a mother to confer immunity to her fetus.
Immunoglobulin M (IgM)
IgM occurs mainly as a pentameric molecule (5 subunits). It is one of the first antibodies on the scene for fighting an infection. Exposure to a pathogen spikes IgM levels, but then IgG antibodies take over. IgM aids in B cell “memory”, so the body responds more quickly to a pathogen when it is exposed again later.
IgM vs IgG vs IgA (Quick Comparison)
| Feature | IgM | IgG | IgA |
|---|---|---|---|
| First produced in infection | Yes | No | No |
| Main location | Blood | Blood and tissues | Mucosal secretions |
| Structure | Pentamer | Monomer | Dimer (secretions) |
| Crosses placenta | No | Yes | No |
| Key role | Early immune response | Long-term immunity | Mucosal defense |
Antibody Subclasses
Some antibody types contain subclasses, which differ slightly in structure and function.
The most important example is IgG.
IgG subclasses
| Subclass | Characteristics |
|---|---|
| IgG1 | Most abundant IgG subclass, important in antiviral and antibacterial immunity |
| IgG2 | Responds strongly to polysaccharide antigens |
| IgG3 | Highly effective at activating complement |
| IgG4 | Often associated with chronic antigen exposure |
IgA also has two subclasses:
• IgA1, common in blood
• IgA2, more resistant to bacterial enzymes and common in mucosal secretions
Class Switching
B cells can change the type of antibody they produce through a process called class switching.
When a B cell first encounters an antigen, it typically produces IgM antibodies. With signals from helper T cells and cytokines, the B cell can switch to producing IgG, IgA, or IgE antibodies.
During class switching:
• The constant region of the heavy chain changes
• The antigen-binding variable region stays the same
Because the binding region does not change, the new antibody recognizes the same antigen but performs a different immune function.
Class switching allows the immune system to tailor the response to the type of infection and the tissue involved.
Why the Immune System Uses Multiple Antibody Types
Different antibody classes allow the immune system to respond effectively in different locations and stages of infection.
Each antibody type has structural features that determine where it operates and how it fights pathogens.
For example:
• IgA protects mucosal surfaces such as the respiratory and digestive tracts.
• IgM responds quickly when the body first encounters a pathogen.
• IgG provides long-term immunity and circulates throughout the bloodstream.
• IgE helps eliminate parasites and triggers allergic responses.
• IgD functions mainly as a receptor that activates B cells.
This specialization allows the immune system to defend both internal tissues and external body surfaces.
Types of Antibodies in Other Species
Different vertebrates produce antibody classes that differ from those in humans.
For example:
• Camelids (camels, llamas, alpacas) produce unusual antibodies that contain only heavy chains. These antibodies are called nanobodies and are widely used in biotechnology.
• Cartilaginous fish, including sharks and skates, produce heavy-chain antibodies similar to camelid antibodies.
• Birds and reptiles produce IgY, which is evolutionarily related to mammalian IgG.
• Bony fish produce specialized antibodies such as IgT and IgZ, which help protect mucosal tissues.
These variations show how the adaptive immune system evolved across vertebrate species.
Common Misconceptions About Antibody Types
IgE only causes allergies.
IgE is famous for causing allergic reactions, but its evolutionary role is defense against parasites such as helminths.
IgM is the most abundant antibody.
IgM is the first antibody produced during infection, but IgG is the most abundant antibody in blood plasma.
All antibodies circulate freely in blood.
Some antibodies, such as IgD, mainly function as receptors attached to B cells rather than circulating freely.
FAQs
Which antibody is most abundant in humans?
IgG is the most abundant antibody in blood plasma, accounting for roughly 75–80% of serum antibodies.
Which antibody appears first during infection?
IgM is usually the first antibody produced during the primary immune response.
Which antibody protects mucosal surfaces?
IgA protects mucosal tissues such as the respiratory, digestive, and reproductive tracts.
Which antibody causes allergic reactions?
IgE triggers allergic reactions by binding to mast cells and basophils.
Which antibody crosses the placenta?
IgG is the only antibody class that crosses the placenta, allowing maternal antibodies to protect the fetus.
References
- Janeway, C. (2001). Immunobiology (5th ed.). Garland Publishing. ISBN 978-0-8153-3642-6.
- Litman, G.W.; Rast, J.P.; et al. (1993). “Phylogenetic diversification of immunoglobulin genes and the antibody repertoire”. Molecular Biology and Evolution. 10 (1): 60–72. doi:10.1093/oxfordjournals.molbev.a040000
- Market, E.; Papavasiliou, F.N. (2003). “V(D)J recombination and the evolution of the adaptive immune system”. PLOS Biology. 1 (1): E16. doi:10.1371/journal.pbio.0000016
- Rhoades, R.A.; Pflanzer, R.G. (2002). Human Physiology (5th ed.). Thomson Learning. ISBN 978-0-534-42174-8.
- Roux, K.H. (October 1999). “Immunoglobulin structure and function as revealed by electron microscopy”. International Archives of Allergy and Immunology. 120 (2): 85–99. doi:10.1159/000024226