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 (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. Depending on whether the antibody is free-floating or bound to another cell, it either attaches to the antigen to tag it for removal or else binds to it so the cell carrying the antibody can destroy the target.
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 binds 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.
|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%|
|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)
Muscosal 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, accounting for 75%-80% of all antibodies in the body. It either tags an antigen for removal or it stimulates the production of toxins or other responses to destroy the pathogen.
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.
Sometimes one antibody isotype switches into another following the activation of B-cells. This lets the cell produce different types of antibodies. Initially, B cells express only IgM and IgD on their cell surface. Activating the cell helps it identify the best type of antibody for eliminating the antigen. So, daughter cells from the B cell produce different antibody isotypes (IgG, IgA, or IgE). During class switching, only the constant region of the antibody heavy chain changes. The variable regions, which are the antigen binding sites, do not change. Cytokines trigger class switching.
Types of Antibodies in Other Species
There are different types of antibodies in nonplacental mammals and in other animals. For example, camelids (camels, llamas, alpacas) only produce antibodies with heavy chains. Sharks and other cartilaginous fish also produce heavy-chain-only antibodies, plus these chains are longer than in mammals. Birds and reptiles produce IgY antibodies, which most closely resemble IgG in mammals. Sharks and skates make IgW, which resembles IgD. Bony fish produce IgT and IgZ, which don’t correspond to mammalian antibodies.
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