Foreign invaders such as bacteria and viruses are constantly attacking our bodies. The immune system produces special proteins known as antibodies to defend itself. Antibodies attach to these invaders and trigger an immune response. In this topic, we will discuss the structure of antibodies, what can they target, and how exactly they protect us.
Antibodies and antigens
Antibodies are proteins produced by cells called lymphocytes in response to foreign substances called antigens. Antibodies recognize and attach to antigens to neutralize them. Each antibody is specific to only one particular antigen and can only attach to that particular antigen.
Antibodies are produced by plasma cells (a type of B cell). Plasma cells develop from B cells when they encounter an antigen. Once they become plasma cells, they begin to produce large quantities of antibodies which bind to the antigen. This process is known as clonal selection (TOPICS: Lymphocytes, Antigen presentation).
The structure of an antibody is like a Y shape, with two arms that come together at a point. Each arm has a heavy chain (H) and a light chain (L). Both the heavy and light chains have specific sequences to recognize the antigen, together they make up the end of each arm and are called the Fab (fragment antigen binding) or variable regions. The heavy chain is longer, and the part that makes up the base of the Y shape is called the Fc (fragment crystallizable/constant) region. This section of the antibody gives it the biological ability to activate other parts of the immune system when the two arms bind to their target molecules on the invader.
The ability of the immune system to produce a wide range of different antibodies is crucial to its success. VDJ recombination is a process that helps create this diversity. It occurs during the development of B cells; during VDJ recombination, the DNA that encodes for antibodies is shuffled around so that each B cell has a unique combination. This process creates millions of different possible combinations, meaning that there are potentially millions of different types of antibody that can be produced. While most combinations will be useless against a given invader, another will be just what's needed to fight off that specific infection. This process increases the chances that at least some B cells will have the right combination of genes to deal with whatever comes their way. Let's discuss what different types of antibodies can do below!
Types and functions
There are five main types of antibodies (or else, immunoglobulins): IgA, IgG, IgM, IgD and IgE . They are distinguished by the type of heavy chain found in the molecule, and each of them has a different function in the immune response.
IgA is mostly found in mucus membranes, such as the lining of the gut and respiratory tract, or secretions (e.g. saliva or tears), where it's presented in a dimeric form. It can attach to and penetrate epithelial surfaces to protect them from bacteria and viruses that enter the body through these tissues. Furthermore, secretory IgA interacts with lysozymes (which are also found in a variety of secreted fluids) to hydrolyze carbohydrates in bacterial cell walls, allowing the immune system to clear the infection.
IgG is the most common antibody in the blood, lymph fluid, cerebrospinal fluid, and peritoneal fluid. It protects against bacterial and viral infections by neutralizing toxins and destroying infected cells. The heavy chains of IgG molecules can bind to receptors found on the surfaces of macrophages, neutrophils, and natural killer cells. The complement system is activated as a result of this interaction. The binding of the Fc portion of IgG to the receptor is an important step in opsonization, which is the process by which phagocytic cells recognize particles for engulfment. Phagocytosis of particles coated with IgG antibodies is an important mechanism through which cells deal with microorganisms. IgG stay in the body for a very long time after being produced; thus, the presence of IgG antibodies usually indicates a previous infection or vaccination.
IgM is also found in blood and lymph fluid. It's generally the first antibody produced in response to an infection. Like IgG, it can neutralize toxins and destroy infected cells. It is responsible for agglutination and cytolytic reactions because, in theory, its structure provides 10 free antigen binding sites and high affinity. IgM is not as flexible as IgG but is essential in complement activation and agglutination. Elevated IgM levels can also indicate a recent infection.
IgD is found on the surface of B cells. It helps B cells detect antigens in the body so they can mount an immune response. However, the exact function of this type remains a mystery.
IgE is found in small amounts in the bloodstream. IgE has an additional domain in its heavy chain that binds to eosinophil, mast cell, and basophil receptors with high affinity. When antigens bind to the Fab region of the IgE attached to these cells, the cells degranulate and release heparin, histamine, proteolytic enzymes, and cytokines. This response causes typical allergic reactions such as mucus secretion, sneezing, coughing, or tear production to remove allergens from the body. In addition, diseases like dermatitis, eczema, urticaria, asthma, and rhinitis cause elevated IgE levels.
Conclusion
Our bodies are constantly under attack from viruses, bacteria, and other foreign invaders. To fight back, we rely on our immune system to produce antibodies. Without antibodies, we would be defenseless against disease–causing microbes. Fortunately, our bodies are very good at making them. Every time we're exposed to a new virus or bacterium, our immune system kicks into gear and produces the specific antibodies we need to fight it off. So the next time you get sick, remember that your body is doing its best to protect you!