5 Classes of Antibodies
The human body is prone to attack by harmful chemicals called toxins
and microorganisms such as bacteria, fungi, parasites and viruses. In
the process of defending itself, the body produces specialized immune
proteins called antibodies which detect the harmful substances called
antigens. The body can also be triggered by certain factors to produce
antibodies against its own cells. An antibody also known as an
immunoglobulin possesses a remarkable ability to combine with every
antigen that gains access to the body and triggers its production
(McGuigan, 2009). The major function of the immune system is the
production of the antibodies. Antibodies are produced by a type of
specialized white blood cells called B-Cell or the B lymphocyte. When a
B-Cell encounters its triggering antigen, it produces many large plasma
cells. Each and every plasma cell is a factory of a certain antibody
(Dugdale, 2009).
All antibodies have similar basic structures. However, the tips of the antibodies are very different and this is the part that interacts with the antigen itself allowing for the production of numerous unique antibodies. Antibodies bind to specific binding sites on the antigen called epitopes. Each antibody is only able to fit into one epitope due to the specificity of its tip. The antigen is immediately recognized by the bodies’ defense mechanism once it has been bound to the antibody (Rank, 2009). After the antigens have been bound to the antibodies, they are then attacked by the killer T-cells. A B-cell also responds immediately to an antigen by binding to it due to its large component of antibodies in its surface. A B-cell absorbs both the antigen and the antibody and processes them into peptides which greatly attract the helper T-cells. The helper T-cells interact with the B-cells making it to split and release copies of antibodies targeting the antigens it had absorbed. Antibodies also prevent the antigens from attacking other body cells by binding to the sites where these antigens uses to bind to the body cells. The reaction between the antigens and the antibodies constitute the humoral immune response. In mammals, there are five classes of antibodies (RrLisa, 2007).
The Five Classes of Antibodies
Immunoglobulin M (IgM)
These antibodies are the largest antibodies in the body. They make up approximately 13% of the serum antibodies. They are the first antibodies to respond to an invading pathogen. They are very important in offering protection in the very early stages of an infection. They tend to stay a lot in the blood stream where they assist in the detection of antigens and killing of bacteria. They have a half life of about five days. The IgM isotype is expressed on the surface of B-cells. IgM is a pentamer and has got 10 epitope binding sites. The Fc portions of this isotype, triggers a complement pathway that leads to the production of IgG, the next class of antibodies that responds to the invasion of a pathogen after the IgM.
Immunoglobulin G (IgG)
There are 4 subtypes of IgG; IgG1 to IgG4. They are the smallest types of antibodies. However, they are produced in large quantities making them the most common antibodies in the body fluids. It provides the majority of the antibody mediated protection against infections. The IgG are rarely produced in the initial stages of an infection. They are produced almost a month after the initial B-cell activation. They are the second type of antibody to be produced in response to a particular infection. While in the blood stream they last over a month and circulate from the blood stream to the tissues easily as they are the only antibodies that can cross the small walls of the blood vessels and reach pathogens that are in the extracellular space. These antibodies have a half life of 7-23 days depending on the subclass. The IgG is a monomer, that is to say it contains a single antibody protein complex and has got two binding sites. Their main function is to bind to the pathogens through specific antigen receptors forming antigen antibody complexes.
The IgG activates a complement system and a cascade of reactions occurs, resulting in the formation of a molecule that easily destroys bacteria cells. The Fc component of the IgG binds to the macrophages and the neutrophils for better phagocytosis. The Fc portion of the IgG binds to the NK (natural killer) cells for antibody dependent cytotoxicity. When the level of the IgG is very high in circulation, a feed mechanism to the B-lymphocytes prevents their activation in order to turn off the production of antibodies (Corbeil et al, 1974). They usually work in the blood and the body tissues. Their main work is to bind to the invading pathogens so that the immune cells such as the macrophage can easily bind and ingest it. They can be passed from mother to child through the placenta. They are the only type of antibodies that can cross the placenta due to their unique Fc component and therefore provide protection to the baby for the first 6-12 months of its life giving time for the baby’s immune system to mature (Vedhara, & Irwin, 2005).
Immunoglobulin A (IgA)
The third isotype of the antibodies is the IgA. Immunoglobulin A has got two subtypes, IgA-1 and IgA-2. They make up about 6% of the serum antibodies. Their half life is approximately 6 days. These antibodies are produced along the surface lining of the body tissues such as in the lungs and in the intestines. IgA antibodies are also produced in body fluids such as the saliva, tears, and mucus. They are produced in the mucosal associated lymphoid tissues (MALT). These isotopes appear as a dimer of 2 “Y”- shaped molecule. It has got 4 epitope binding sites. They also have a secretory component that produces chemicals that prevent it from being digested by the digestive enzymes (Blaese & Winkelstein, 2007).
The Fc portions of the IgA bind to specific sites of the mucous components and enable the mucous to trap the pathogens easily. The IgA activate the lectin complement pathway and the alternative complement pathway but not the classical complement pathway (Stanier, 1986). These antibodies protect the body against infections to the internal surfaces that are exposed to the environment such as the respiratory tract and the intestines. These antibodies are passed from mother to child through breast milk. This helps in preventing the colonization of the baby’s gastrointestinal tract form colonization by harmful pathogens. They also provide the child with passive immunity against pathogens the mother has encountered (Vedhara, & Irwin, 2005).
Immunoglobulin E (IgE)
The fourth class of antibodies is the IgE. These are normally found in trace amounts in the body approximately .0002% and are very important in allergic reactions. They have a half life of about 2 days. Through their Fc portions, most of the immunoglobulin-E are bound to the basophils and the mast cells mediating many of the allergic reactions. They promote inflammation of the inner surface lining enabling IgG, complement proteins and leucocytes to enter this tissue. The connection of cell bound IgE and an antigen triggers the discharge of vasodilators such as histamine for an inflammatory response. Their main function is to cause the body to react to foreign particles such as the pollen, fungus spores, and animal fur. They may also cause allergic reactions to some medicines, some foods, milk and poisons. The IgE cause allergic reactions due to its ability to trigger the granulocytes to release their toxic chemicals when antibodies interact with specific antigens (Lloyd, 2008).
Immunoglobulin D (IgD)
The last class of antibodies is the IgD. It has got two sub classes, IgD-1 and IgD-2. They make up about 2% of the serum antibodies. IgD is a monomer and has got 2 epitope binding sites. They are present on the surface of the B-Cells which have not been activated by an antigen. Their work is to provide signals to the B-cells after they have matured fully in the spleen. IgG controls the activation and suppression of the B-lymphocytes. They also play a very vital responsibility in elimination of B-lymphocytes generating self reactive auto antibodies.
These antibodies defend the body through various ways which include; opsonization, membrane attack complex (MAC) cytolysis, antibody dependent cellular cytotoxicity (ADCC), neutralization of exotoxins, neutralization of viruses, prevention of bacterial adherence to host cells, agglutination of microorganisms, and immobilization of bacterial protozoans (Kaiser, 2007).
Conclusion
When a pathogen enters the body for the first time, disease symptoms appear as the body makes antibodies against that specific pathogen. Subsequent entry of the pathogen triggers the release of previously created antibodies as a result of the simulation of immune memory. This clarifies the fact that a person may have so many symptoms of a disease and not even be aware of the exposure to the antigen. Immunization process utilizes this fact, where an inactivated or dead pathogen is introduced into the body to trigger the production of antibodies against it. These antibodies are then retained in circulation awaiting future invasion by the pathogen.
All antibodies have similar basic structures. However, the tips of the antibodies are very different and this is the part that interacts with the antigen itself allowing for the production of numerous unique antibodies. Antibodies bind to specific binding sites on the antigen called epitopes. Each antibody is only able to fit into one epitope due to the specificity of its tip. The antigen is immediately recognized by the bodies’ defense mechanism once it has been bound to the antibody (Rank, 2009). After the antigens have been bound to the antibodies, they are then attacked by the killer T-cells. A B-cell also responds immediately to an antigen by binding to it due to its large component of antibodies in its surface. A B-cell absorbs both the antigen and the antibody and processes them into peptides which greatly attract the helper T-cells. The helper T-cells interact with the B-cells making it to split and release copies of antibodies targeting the antigens it had absorbed. Antibodies also prevent the antigens from attacking other body cells by binding to the sites where these antigens uses to bind to the body cells. The reaction between the antigens and the antibodies constitute the humoral immune response. In mammals, there are five classes of antibodies (RrLisa, 2007).
The Five Classes of Antibodies
Immunoglobulin M (IgM)
These antibodies are the largest antibodies in the body. They make up approximately 13% of the serum antibodies. They are the first antibodies to respond to an invading pathogen. They are very important in offering protection in the very early stages of an infection. They tend to stay a lot in the blood stream where they assist in the detection of antigens and killing of bacteria. They have a half life of about five days. The IgM isotype is expressed on the surface of B-cells. IgM is a pentamer and has got 10 epitope binding sites. The Fc portions of this isotype, triggers a complement pathway that leads to the production of IgG, the next class of antibodies that responds to the invasion of a pathogen after the IgM.
Immunoglobulin G (IgG)
There are 4 subtypes of IgG; IgG1 to IgG4. They are the smallest types of antibodies. However, they are produced in large quantities making them the most common antibodies in the body fluids. It provides the majority of the antibody mediated protection against infections. The IgG are rarely produced in the initial stages of an infection. They are produced almost a month after the initial B-cell activation. They are the second type of antibody to be produced in response to a particular infection. While in the blood stream they last over a month and circulate from the blood stream to the tissues easily as they are the only antibodies that can cross the small walls of the blood vessels and reach pathogens that are in the extracellular space. These antibodies have a half life of 7-23 days depending on the subclass. The IgG is a monomer, that is to say it contains a single antibody protein complex and has got two binding sites. Their main function is to bind to the pathogens through specific antigen receptors forming antigen antibody complexes.
The IgG activates a complement system and a cascade of reactions occurs, resulting in the formation of a molecule that easily destroys bacteria cells. The Fc component of the IgG binds to the macrophages and the neutrophils for better phagocytosis. The Fc portion of the IgG binds to the NK (natural killer) cells for antibody dependent cytotoxicity. When the level of the IgG is very high in circulation, a feed mechanism to the B-lymphocytes prevents their activation in order to turn off the production of antibodies (Corbeil et al, 1974). They usually work in the blood and the body tissues. Their main work is to bind to the invading pathogens so that the immune cells such as the macrophage can easily bind and ingest it. They can be passed from mother to child through the placenta. They are the only type of antibodies that can cross the placenta due to their unique Fc component and therefore provide protection to the baby for the first 6-12 months of its life giving time for the baby’s immune system to mature (Vedhara, & Irwin, 2005).
Immunoglobulin A (IgA)
The third isotype of the antibodies is the IgA. Immunoglobulin A has got two subtypes, IgA-1 and IgA-2. They make up about 6% of the serum antibodies. Their half life is approximately 6 days. These antibodies are produced along the surface lining of the body tissues such as in the lungs and in the intestines. IgA antibodies are also produced in body fluids such as the saliva, tears, and mucus. They are produced in the mucosal associated lymphoid tissues (MALT). These isotopes appear as a dimer of 2 “Y”- shaped molecule. It has got 4 epitope binding sites. They also have a secretory component that produces chemicals that prevent it from being digested by the digestive enzymes (Blaese & Winkelstein, 2007).
The Fc portions of the IgA bind to specific sites of the mucous components and enable the mucous to trap the pathogens easily. The IgA activate the lectin complement pathway and the alternative complement pathway but not the classical complement pathway (Stanier, 1986). These antibodies protect the body against infections to the internal surfaces that are exposed to the environment such as the respiratory tract and the intestines. These antibodies are passed from mother to child through breast milk. This helps in preventing the colonization of the baby’s gastrointestinal tract form colonization by harmful pathogens. They also provide the child with passive immunity against pathogens the mother has encountered (Vedhara, & Irwin, 2005).
Immunoglobulin E (IgE)
The fourth class of antibodies is the IgE. These are normally found in trace amounts in the body approximately .0002% and are very important in allergic reactions. They have a half life of about 2 days. Through their Fc portions, most of the immunoglobulin-E are bound to the basophils and the mast cells mediating many of the allergic reactions. They promote inflammation of the inner surface lining enabling IgG, complement proteins and leucocytes to enter this tissue. The connection of cell bound IgE and an antigen triggers the discharge of vasodilators such as histamine for an inflammatory response. Their main function is to cause the body to react to foreign particles such as the pollen, fungus spores, and animal fur. They may also cause allergic reactions to some medicines, some foods, milk and poisons. The IgE cause allergic reactions due to its ability to trigger the granulocytes to release their toxic chemicals when antibodies interact with specific antigens (Lloyd, 2008).
Immunoglobulin D (IgD)
The last class of antibodies is the IgD. It has got two sub classes, IgD-1 and IgD-2. They make up about 2% of the serum antibodies. IgD is a monomer and has got 2 epitope binding sites. They are present on the surface of the B-Cells which have not been activated by an antigen. Their work is to provide signals to the B-cells after they have matured fully in the spleen. IgG controls the activation and suppression of the B-lymphocytes. They also play a very vital responsibility in elimination of B-lymphocytes generating self reactive auto antibodies.
These antibodies defend the body through various ways which include; opsonization, membrane attack complex (MAC) cytolysis, antibody dependent cellular cytotoxicity (ADCC), neutralization of exotoxins, neutralization of viruses, prevention of bacterial adherence to host cells, agglutination of microorganisms, and immobilization of bacterial protozoans (Kaiser, 2007).
Conclusion
When a pathogen enters the body for the first time, disease symptoms appear as the body makes antibodies against that specific pathogen. Subsequent entry of the pathogen triggers the release of previously created antibodies as a result of the simulation of immune memory. This clarifies the fact that a person may have so many symptoms of a disease and not even be aware of the exposure to the antigen. Immunization process utilizes this fact, where an inactivated or dead pathogen is introduced into the body to trigger the production of antibodies against it. These antibodies are then retained in circulation awaiting future invasion by the pathogen.
