Properties and Function of different classes of Antibodies
Antibodies are specialized proteins that are produced by the immune system to fight against foreign substances that enter the body, such as bacteria, viruses, fungi, allergens, and toxins. These substances are called antigens and they have distinct molecules on their surface that can be recognized by antibodies. Antibodies bind to antigens with a specific shape and either destroy them directly or mark them for destruction by other immune cells and proteins.
Another word for antibody is immunoglobulin (Ig). There are five classes of antibodies or immunoglobulins: IgG, IgM, IgA, IgD, and IgE . They differ in their heavy chains, which have different amino acid sequences and constant domains. The heavy chains are labeled by Greek letters: gamma (γ), mu (μ), alpha (α), delta (δ), and epsilon (ε), respectively. The light chains are either kappa (κ) or lambda (λ) and they are identical within each antibody class.
Each class of antibody has a distinct structure, location, and function in the immune system. They can be found in various fluids and tissues of the body, such as blood, saliva, tears, mucus, breast milk, and intestinal secretions. They can also be expressed on the surface of B cells, which are specialized white blood cells that produce antibodies. Some antibodies can cross the placenta and protect the developing fetus from infections. Some antibodies can activate the complement system, which is a group of proteins that enhance the immune response. Some antibodies can mediate allergic reactions or protect against parasitic infections.
In this article, we will discuss the properties and functions of each class of antibody in more detail. We will also explain how they work together to provide immunity against various pathogens and diseases.
Immunoglobulin A (IgA) is the predominant immunoglobulin in various secretions, such as colostrum, saliva, tears, bronchial secretions, nasal mucosa, prostatic fluid, vaginal secretions, and mucous secretions of the small intestine. It is also present in serum, but at a lower concentration than IgG. IgA has two subclasses: IgA1 and IgA2, which differ in their heavy chain structure and susceptibility to proteolytic cleavage by bacterial enzymes.
IgA has several functions that contribute to the immune defense of mucosal surfaces and prevent the entry of pathogens into the body. Some of these functions are:
- Blocks antigen uptake and bacterial or viral attachment. IgA binds to antigens that leak across an epithelium and transports them back across to prevent their entry into the circulation. IgA also prevents the attachment of bacteria or viruses to the mucosal epithelium by forming a protective layer or coating on the surface.
- Limits inflammation induced by classical pathway complement activation. IgA does not activate the classical pathway of complement, but instead activates the alternative pathway, which is less inflammatory and more efficient at clearing microbes. IgA also inhibits the binding of C1q to IgG or IgM, thus preventing the activation of the classical pathway by other immunoglobulins.
- Promotes microbial destruction through antibody-dependent cellular cytotoxicity (ADCC) by binding to leukocyte receptors. IgA binds to Fc receptors on various leukocytes, such as neutrophils, eosinophils, monocytes, macrophages, and natural killer cells, and triggers their cytotoxic activity against microbes coated with IgA.
- Secretory IgA can play an important first line of defense in antigen clearance by binding to antigens that leak across an epithelium and transporting them back across to prevent their entry. Secretory IgA is a dimeric form of IgA that is produced by plasma cells in mucosal tissues and transported across the epithelium by a specialized receptor called polymeric immunoglobulin receptor (pIgR). Secretory IgA has an additional component called secretory component (SC), which is derived from the cleavage of pIgR and protects IgA from degradation by proteases in the secretions.
IgM is the largest and most complex of the immunoglobulin classes. It is composed of five immunoglobulin subunits (monomeric subunits, IgMs) and one molecule of J chain, which links the subunits together. The J chain also facilitates the transport of IgM across epithelial cells and the binding of IgM to some receptors. IgM has two subclasses: IgM1 and IgM2, which differ in their µ chains. IgM1 consists of µ1 and IgM2 consists of µ2 chains.
IgM has several important functions in the immune system:
- It is the first immunoglobulin to be produced in response to an antigen, giving the primary immune response. It can bind to antigens with low affinity and high avidity, meaning that it can recognize a wide range of epitopes and form large immune complexes.
- It is highly efficient in activating the classical complement pathway, which leads to the lysis of pathogens, opsonization, inflammation, and clearance of immune complexes.
- It is expressed on the surface of B cells, where it serves as an antigen receptor. It signals the activation, proliferation, and differentiation of B cells into plasma cells or memory cells.
- It acts as an agglutinin, meaning that it can clump together antigens such as bacteria or red blood cells. This can prevent the spread of infection or facilitate phagocytosis by macrophages.
- It can also act as an opsonin, a lysin, or a complement-fixing antibody, depending on the type of antigen and the effector mechanism involved.
IgM is mainly found in the blood and lymphatic fluid, where it confers protection against extracellular pathogens that are present in these fluids. It can also be secreted into mucosal surfaces as a pentamer or as a dimer with a secretory component. The secretory component protects IgM from degradation by proteases and helps it bind to mucosal epithelial cells.
Immunoglobulin E (IgE) is a type of antibody that mediates the type I immediate hypersensitivity (atopic) reaction. This is a rapid and exaggerated immune response that occurs when an individual is exposed to an allergen, such as pollen, dust mites, animal dander, or food proteins. IgE binds to mast cells and basophils, which are specialized cells that release histamine and other inflammatory mediators when triggered by IgE. These mediators cause symptoms such as itching, sneezing, wheezing, swelling, and anaphylaxis.
IgE is mostly found extravascularly in the lining of the respiratory and intestinal tracts, where it can encounter potential allergens. IgE levels are normally very low in the serum, but they can increase dramatically in individuals with allergic diseases such as asthma, allergic rhinitis, atopic dermatitis, and food allergies.
IgE also plays an important role in protection against parasitic infections, especially helminths (worms). IgE can bind to antigens on the surface of parasites and activate eosinophils, which are white blood cells that can kill parasites by releasing toxic granules. IgE can also enhance the phagocytosis of parasites by macrophages and neutrophils.
IgE is produced by plasma cells that are derived from B cells that have been activated by T helper 2 (Th2) cells. Th2 cells secrete cytokines such as interleukin-4 (IL-4), interleukin-5 (IL-5), and interleukin-13 (IL-13), which promote the differentiation and proliferation of IgE-producing B cells. The production of IgE is also regulated by other factors such as genetic predisposition, environmental exposure, and epigenetic modifications.
IgE has a unique structure among immunoglobulins. It consists of two heavy chains (ε chains) and two light chains (κ or λ chains). The ε chains have four constant domains (Cε1-Cε4) and one variable domain (Vε). The Cε3 domain contains a binding site for the high-affinity IgE receptor (FcεRI), which is expressed on mast cells and basophils. The Cε4 domain contains a binding site for the low-affinity IgE receptor (FcεRII or CD23), which is expressed on B cells, eosinophils, macrophages, and dendritic cells. The binding of IgE to these receptors can modulate the function of these cells in various ways.
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