An antigen is a molecule that is part of an object that is foreign to the body. The body uses antibodies to recognize the foreign object by its antigens and stimulates an immune response, activating white blood cells to produce more antibodies and other immune pathways. Antigens can be proteins or sugars that are located on the outer surfaces of pathogenic cells. All cells have antigens including the ones inside the body, bacteria, and even viruses. The antibodies produced by the immune system are custom-fitted to the antigen that initially stimulated the immune response. The antibodies have an antigen recognition site (paratope) that is highly specific affinity for a region on the antigen called the epitope.
There are three main types of antigen
The three broad ways to define antigen include exogenous (foreign to the host immune system), endogenous (produced by intracellular bacteria and virus replicating inside a host cell), and autoantigens (produced by the host). Normally, autoantigens would not elicit an immune response, but a sensitized immune system will mount an attack that result in the development of an autoimmune disease. A commonly known name for the presence of autoantigens is allergies.
Examples of antigens
Blood group antigens
Perhaps some of the most familiar antigens to most people are the red blood cell antigens. These are proteins and sugars that coat the surface of red blood cells. The A, B, and O antigens are sugars, which are also called glycans. The Rh antigen, which is either present (+) or absent (-) is a protein. When these antigens are defined together the result is blood type (ex: O+, AB+, B-, and so on). Many people died as a result of mismatched blood transfusions before these antigens were first studied and defined. The reason for those deaths was a severe immune response against the transfused red blood cells. For example, if an O- patient received a blood transfusion from an O+ patient, then the recipient’s immune system would recognize that Rh protein on the transfused cells as a foreign threat and would mount an attack to destroy those cells. The same thing happens if the glycans A,B, and O are mismatched. Interestingly, the blood type of a pregnant mother and her fetus are routinely checked in modern healthcare settings; if a mismatch is detected then the mother will receive a medication that will prevent her immune system from producing antibodies against that antigen. This is important to prevent immune reactions against mismatches in future pregnancies.
The influenza virus is particularly dangerous to public health because of it high rate of mutation. The antigens on the surfaces of the influenza virus are glycoproteins called hemagglutinin and neuraminidase. The hemagglutinin antigen is capable of undergoing rapid changes called antigenic drift that enable the virus to escape detection by antibodies. Small amino acid substitutions in the glycoprotein can change the epitope enough that the antibody doesn’t recognize the antigen anymore. This is why there is a new flu vaccine every year. The vaccine developers track influenza mutation patterns around the globe and try to predict what the new strains will look like in the upcoming flu season. The vaccine is a cocktail of three or four influenza strains. The benefit of getting a flu shot every year is that the immune system is able to start to develop a “library” of antibodies against multiple influenza antigens. This improves the chances of having an antibody that will recognize the virus at first entry into the body and fight it before the infection becomes too strong.
The interaction between antigen and antibody is so specific that even small changes in the antigen protein structure can render a previously matched antibody ineffective. The adaptive immune system is designed to battle the inevitable antigenic mutations that occur in pathogens, and this is why it is so powerful. However, when the immune system starts to produce autoantibodies against autoantigens it can lead to autoimmune disease. Some scientists have hypothesized that modern humans live in such “clean” environments (free of many pathogens that our ancestors routinely encountered) that our immune systems don’t have enough exogenous antigens to fight, and instead become sensitized to autoantigens. The increased rates of autoimmune disease are indeed concerning and it may be many years before we truly begin to understand the underlying cause of this rise.
An article about antigens would not be complete without discussing antibodies. An antibody is a molecule of the immune system that is shaped like a Y. The “arms” are where the paratope is, the part that binds the antigen at its epitope. The strength of this interaction is determined not only by the amino acids at this section of the antibody and antigen but also by the structure and shape of the area. The key amino acids of the antigen that bind the antibody may not be in a straight row but bind along curves and folds. The strength of this interaction can be described by affinity, given in molarity. The lower the molarity of antibody needed to bind the antigen, the higher the affinity or strength of the interaction is. Antibodies can become higher affinity towards antigens they are presented with repeatedly by undergoing a process called affinity maturation. The antibody-antigen interaction is incredibly important for the immune system. It is also useful in clinical/research settings in techniques like ELISA.
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