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This is the third edition of Through the Microscope. A new edition has just been published. Please go to the Table of contents for the fourth edition
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We will now spend a bit more time looking at the structure and function of antibodies to give you a more in-depth understanding of their different types and roles in the immune response.
Antibodies have the interesting challenge of needing to respond to a wide array of antigens, yet still be recognizable to the immune system. They therefore, need to interact with two different types of macromolecules, antigens and parts of the immune system. If antibodies were too similar to each other in structure, they would not be able to recognize many antigens, but if they were too different, they would not have a consistent enough structure to react with various immune components. Antibodies solve this dilemma by having two regions, a highly variable region that recognizes the antigens and constant regions that interact with the immune system.
Plasma cells synthesize a number of different types of antibodies and these serve different functions for the cell. Antibodies are proteins made up of two light chains and two heavy chains, see Figure 15-1. The heavy chain determines the type of antibody class and is bound to the light chain by sulfhydryl linkages. Each chain has a constant region that is similar amongst all antibodies. The constant region interacts with the rest of the immune system to facilitate the immune response. Each chain also has a variable region containing a high degree of amino acid variability that is unique to each antibody and this is the part that binds to the antigen. The proteins wrap together and form a Y structure with the two prongs of the Y containing the variable (Fab) regions and the bottom stem of the Y containing the constant (Fc) region.
The different classes of antibodies are defined by the structure of the constant region of the heavy chain. Each heavy chain gives the class of antibody certain properties. Here we first introduce the properties of these antibody classes and then describe the various roles antibodies play in the inactivation of pathogenic microorganisms.
While billions of different variable regions are made, the general structure of antibodies falls into just five classes and this is based upon the type of heavy chain present in the antibody as shown in Figure 15-16. Immunoglobulin G (IgG) is the most abundant circulating antibody, making up 80% of the total antibodies and 75% of that found in serum. It contains a single antibody protein complex, with two heavy chains and two light chains. IgG is the second type of antibody synthesized in response to an infection and is the only antibody that can pass through the wall of small blood vessels to access antigens present in the extracellular spaces. It is also the only antibody capable of crossing the placenta in humans, where it confers the mother's immunity onto the fetus and newborn. This immunity protects a baby for the first 6-12 months of its life and allows it time for its own immune system to mature. IgG is particularly effective at attacking extracellular viruses and protein toxins and is also capable of activating the classic pathway of the complement cascade. It helps to prevent the systemic spread of infection and facilitates recovery from many infections. Finally, IgG is the antibody that serves as an efficient handle for phagocytes, allowing phagocytes to bind to a pathogen through Fc receptors on IgG and rapidly phagocytize a pathogen.
Figure 15.16 Different Antibody types . There are five different antibodies classes that are formed in the cell. See the text for more details.
IgM is the largest antibody, with five Y structures being joined by their Fc regions in a circular configuration. A J chain (another polypeptide) links the five antibodies together. IgM is present in serum, making up about 10 % of antibodies in the blood. The presence of its ten antigen reactive sites helps agglutinate or clump antigens (see the explanation of this term in the next section), making it easier for the immune system to eliminate them. IgM is more efficient than IgG at activating the complement pathway. IgM is synthesized by plasma cells early in a primary infection and is very important in slowing or stopping the spread of a pathogen during the initial stages of an illness. IgM is also found on mature B cells in a monovalent form, where it serves as a receptor.
IgA is present in serum, mucus, saliva, tears, sweat and milk. Two subclasses with different heavy chains are made, IgA1 and IgA2. IgA1 is synthesized in the bone marrow and makes up most of the serum IgA. IgA2 is synthesized by B cells present in MALT. The antibodies are synthesized as dimers that are joined by a short J chain polypeptide. As the secreted IgA2 passes through the intestinal epithelium, a second secretory protein attaches. Dimerization and binding of the J and secretory proteins make IgA more resistant to proteases present in the environments that it protects. IgA in breast milk interferes with the colonization of the GI tract by harmful microorganisms in the first few months of life. The mother's IgA in the GI tract of newborns keeps these pathogens at low populations, preventing them from causing serious disease, but still allowing the stimulation of the infant's own immune system. The newborn thus develops its own immunity while being partially protected by the mother. IgA molecules do not activate the classical complement pathway, but may activate the alternative complement pathway.
IgE is a monomeric antibody that accounts for only 0.002 % of the total serum antibodies. Almost all IgE is bound to tissue cells, especially mast cells and eosinophils in various parts of the body. Contact of IgE with antigen leads to release of a set of signal molecules from the mast cells, which effectively recruits various agents of the immune response to fight the infection. IgE and MALT serve to detect penetrating pathogens and amplify the immune response in an area leading to the repulsion of the invader. Antigen reactions with IgE are also responsible for atopic allergic reactions (e.g., hives, asthma, hay fever etc.)
IgD is found on the surface of B-lymphocytes and together with monomeric IgM, serves as antigen receptor for the activation of B cell as described previously. IgD is monovalent.[Prev] | [Next]