Antibody classes and their functions (Part 3- Antibody Basics)
Welcome to the 3rd part of the 13-part series on Antibody basics. You can read the first and second parts here to understand what are antibodies and their structure.
There are five different classes of antibodies: IgG, IgM, IgA, IgE, and IgD. Each class of antibody is distinguished by the unique amino acids present in the constant region of the heavy chain that confer structural and functional properties to the antibodies.
Immunoglobulin G or IgG
First is Immunoglobulin G, designated as IgG. It is the most abundant antibody in serum and constitutes about 75–80% of the immunoglobulin serum. IgG molecule consists of two 𝛾 heavy chains and either two κ light chains or two ƛ light chains. But can never have both of the light chains.
There are four different IgG subclasses: IgG1, IgG2, IgG3, and IgG4 that are distinguished by subtle differences in amino acid sequences in the constant region of 𝛾 heavy chains. Besides differences in amino acid sequences, few structural differences distinguish one subclass from another. These are differences:
· in the size of the hinge region
· And the number of interchain disulfide bonds between the heavy chains. For instance, IgG1 has two interchain disulfide bonds; IgG2 and IgG4 have four interchain disulfide bonds, whereas IgG3 has 11 interchain disulfide bonds.
Immunoglobulin G has many biological roles.
The differences in amino acids between subclasses of IgG also affect their biological activity. For instance,
- IgG is the only class of Ig that can cross the placenta in humans and enter the fetal circulation, and it is mainly responsible for the protection of the newborn during the first months of life. The transfer of IgG from mother to fetus is a form of passive immunization, which means the acquisition of immunity by receiving preformed antibodies of the mother by the fetus rather than by active production of antibodies after exposure to antigen. Among all the subclasses, IgG1, IgG3, and IgG4 can readily cross the placenta and play an essential role in protecting the fetus.
- Another important role of IgG is that IgG molecules can react with Fcγ receptors present on the surface of macrophages, neutrophils, and natural killer cells. These cells phagocytose the particles or pathogens coated with IgG antibodies and thus is a vital mechanism that cells use to kill the microorganisms. This process is called opsonization. Among the subclasses of IgG, IgG1 and IgG3 bind with high affinity to Fc receptors and are most efficient in mediating opsonization, whereas IgG4 antibody has an intermediate affinity and IgG2 antibody has an extremely low affinity for Fc receptors.
- The third important role of IgG is that it activates the complement system. The complement system comprises proteins that can perforate the cell membrane of the microorganisms, thus killing them. The collaboration between the antibody and the complement system is essential for the inactivation, removal, and killing of pathogens. The complement system is discussed in detail in the next Part 4. Among the subclasses of IgG, IgG3 is the most effective complement activator, followed by IgG1. On the other hand, IgG2 is less efficient in activating the complement system, whereas IgG4 cannot activate the complement system.
Immunoglobulin A or IgA
The IgA antibody constitutes only 10–15% of total serum immunoglobulins. But is the predominant immunoglobulin found in body secretions such as breast milk, tears, saliva, and mucous membranes of the respiratory, digestive, and genitourinary systems. Structurally IgA molecule consists of two α heavy chains and either two κ light chains or two ƛ light chains. There are two different IgA subclasses IgA1 and IgA2 that are distinguished by subtle differences in amino acid sequences in the constant region of α heavy chains. However, besides differences in amino acid sequences, few structural differences distinguish one subclass from another. These are differences in the size of the hinge region.
· The hinge region of IgA1 is longer than that of IgA2. The IgA1 hinge region consists of 16–18 amino acids, whereas the IgA2 hinge region contains around 5 amino acids.
· The extended hinge region of IgA1 also has glycosylation sites.
Based on their presence in serum and body secretions, IgA can be divided into two forms:
· serum IgA
· and secretory IgA (sIgA)
Serum IgA is predominantly found in serum. In contrast, the secretory IgA is present in body secretions like breast milk, saliva, tears, and mucous membranes.
Additionally, serum IgA generally exists as a monomer, and its polymeric form is rarely found. On the other hand, secretory IgA occurs in polymeric form and consists of either dimer or tetramer. The IgA monomers are held together by Fc-linked polypeptide called the joining chain designated as J chain. The J chain is disulfide-bonded to the carboxyl-terminal cysteine residue of the two subunits of polymeric IgA. The function of the J chain is to facilitate the polymerization of the monomers to form polymeric IgA.
IgA also contains another polypeptide called the secretory component.
The secretory component consists of 5 Ig-like domains that bind to the Fc region of IgA dimer. The interaction is stabilized by a disulfide bond between the fifth domain of the secretory component and one of the chains of dimeric IgA.
Polymeric IgA secreting plasma cells get concentrated along the mucous membrane surfaces. IgA binds to the polymeric Ig receptor (or poly Ig receptor), which recognizes and interacts with the J chain of polymeric IgA antibodies. The poly Ig receptor is expressed on the epithelial cells of the mucous membrane.
Once the polymeric IgA is bound to poly Ig receptor, endocytosis of receptor-IgA complex occurs. After endocytosis, cleavage of the poly Ig receptor occurs. The cleaved poly Ig receptor becomes the secretory component of IgA, and then the IgA bound with the secretory component is released into the mucous secretions. The released IgA antibody is called the secretory IgA (sIgA) antibody. The secretory component of sIgA is essential as it masks the protease cleavage sites in the hinge region of the IgA molecule, thus protecting immunoglobulin from getting degraded by proteolytic enzymes and allowing the polymeric IgA to exist longer in the environment.
Secretory IgA serves a vital function at mucous membrane surfaces, which are the potential entry sites of most pathogenic organisms. The binding of secretory IgA to the membranes of pathogens prevents their attachment to the mucosal membranes, thus inhibiting viral and bacterial infection. For instance, secretory IgA provides defense against bacteria like Salmonella, Vibrio cholerae, and viruses such as polio, influenza, etc.
Breast milk containing secretory IgA helps protect the newborn against infections during the initial months of their life and provides them the required immunity until their own immune system becomes fully functional.
Immunoglobulin M or IgM
Of the five major classes of immunoglobulins, the immunoglobulin M (IgM) antibody is the largest antibody and is the first immunoglobulin class to be produced in a primary response to an antigen. Also, it is the first immunoglobulin to be synthesized by neonates. Structurally IgM molecule consists of two µ heavy chains and either two κ light chains or two ƛ light chains. It accounts for 5–10% of the total serum immunoglobulins and has two forms: monomeric IgM and pentameric IgM.
As a monomer, IgM is present on the surface of B cells. Thus, monomeric IgM is called membrane-bound IgM designated as mIgM. In contrast, the IgM secreted by plasma cells is in the pentameric form. The pentameric form of IgM is present in blood and body secretions.
The five monomer subunits of pentameric IgM are arranged with their Fc regions in the center of the pentamer and the ten antigen-binding sites on the periphery of the molecule. The five monomer units are held together by disulfide bonds that link their carboxyl-terminal, i.e., Cµ4 and the Cµ3 heavy chain domains. Pentamer IgM contains an additional Fc-linked polypeptide called the joining chain designated as J chain, similar to that found in the case of IgA, and is added just before the secretion of the pentamer.
Since each original monomer of immunoglobulin has two antigen-binding sites, therefore each pentameric complex of IgM should be able to bind to 10 identical antigens simultaneously. However, although an IgM molecule can bind to 10 antigen molecules, because of lack of flexibility in the hinge region and steric hindrance that occurs on larger antigens, only five or fewer molecules of antigens can bind simultaneously.
Due to its high valency, pentameric IgM is more efficient than other antibody classes in binding antigens with many repeating epitopes such as antigens of red blood cells (RBCs) and viral particles. For example, IgM antibodies are mainly responsible for the clumping or agglutination of red blood cells if the recipient of a blood transfusion receives blood that is not compatible with their blood type. Interestingly it takes 100 to 1000 times more IgG molecules than IgM to achieve the same level of agglutination.
Similarly, less IgM than IgG is required to neutralize viral infectivity.
Additionally, IgM is the most efficient antibody in activating the complement system. Since the complement activation requires at least two Fc regions in close proximity, the pentameric structure of a single molecule of IgM fulfills this requirement (discussed in Part 4).
The J chain present allows IgM to bind to receptors on epithelial cells on the mucous membrane, which transport it across the epithelial membrane to enter the mucous secretions. The pentameric IgM is transported to mucous secretions through a phenomenon similar to that of IgA that has been already discussed. However, because of its large size, IgM does not diffuse well. Therefore, it is found in very low concentrations in the mucous secretions and is also restricted from crossing the placenta. Although IgM is present in the body secretion, the predominant immunoglobulin class in the secretions is IgA.
Immunoglobulin E or IgE
Structurally IgE is a monomer consisting of two ε heavy chains and either two κ light chains or two ƛ light chains. IgE is found in trace amounts in serum, with its concentration in serum being around 0.3 µg/ml. It constitutes less than 1% of serum immunoglobulins. IgE antibody does not participate in complement activation, opsonization, etc., but it plays a crucial role in allergic reactions and defense against parasitic worms.
IgE in defense against parasitic worms such as helminths etc.: When there is an invasion by parasitic worms, the T helper cells secrete the cytokines such as IL-4 and stimulate B cells to secrete IgE antibodies.
The IgE antibodies then coat the surface of the parasitic worm by binding to its surface antigens. These bound IgE antibodies are then recognized by specific Fcε receptors present on the eosinophils, which bind to the Fc region of IgE bound to worms.
Once bound, the eosinophils undergo translocation of their granules to the plasma membrane, followed by the release of contents of the granules to the extracellular environment. This release of granular content is known as degranulation. The released granule content then destroys the parasitic worms.
Role of IgE in allergic reactions: IgE antibodies also mediate allergic reactions in the body that are responsible for the symptoms of asthma, hay fever, hives, and anaphylactic shock. Some people’s immune system is sensitive to some substances such as pollen, nuts, dust, etc. These substances are known as allergens. These substances are harmless to most people, but the immune system of people allergic to them recognizes them as dangerous. For example, let’s assume that a person is allergic to pollen. On his first exposure to pollen, the B cells of this allergic person get activated. Once activated, the B cells then differentiate into IgE-producing plasma cells.
These IgE antibodies then attach themselves to the Fc receptors on the membranes of basophils and mast cells by their Fc regions. Now, the antigen-binding sites of these IgE antibodies are free. When there is a second exposure of the allergic person to allergen pollen, the allergen binds to the IgE antibodies attached to Fc receptors of mast cells and basophils. This binding of allergen to IgE antibodies results in crosslinking of IgE antibodies.
This crosslinking then sends the signal to the basophils and mast cells to translocate their granules to the plasma membrane and release the granule contents to the extracellular environment by a process known as degranulation. As a result of degranulation, there is a rapid release of preformed active mediators from the granules, such as histamine, heparin, etc.
These mediators act on a person’s eyes, nose, throat, lungs, skin, or gastrointestinal tract, causing allergy symptoms like watery eyes, itching, sneezing, runny nose, rashes, abnormal heart rate, etc. Thus, the elevated levels of IgE antibodies in the body also have a diagnostic significance. It indicates the possibility of either allergic reaction or parasitic infection.
Immunoglobulin D or IgD
IgD antibody constitutes about 0.25% of the total immunoglobulins in serum. Structurally IgD is a monomer. It has two δ heavy chains and either two κ light chains or two ƛ light chains.
IgD can both be membrane-bound and secreted. IgD, together with IgM, is the major membrane-bound antibody expressed by the mature B cells. IgD signals the B cells to get activated. By being activated, B cells are ready to take part in the body’s defense as part of the immune system. The exact function of IgD antibodies is still unclear. Some studies have also suggested that secreted IgD has essential roles in respiratory mucosal immune defense. It binds to microbial virulence factors as well as pathogenic respiratory bacteria and viruses.
Few studies have also suggested that IgD may have a role in allergic responses as well. Although the nature of the IgD receptor remains elusive, the cross-linking of IgD on basophils stimulates the release of immune-activating, proinflammatory, and antimicrobial mediators.
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