Antigens particularly activate the lymphocytes (the white blood cells in our body that combats infections). Generally speaking, there are two main divisions of antigens - autoantigens (also called self-antigens) and foreign antigens (also known as heteroantigens). Autoantigens have their origin within the body. Usually, our body can differentiate between self and substances that do not belong to self (non-self). However, people suffering from autoimmune problems the bodily substances themselves stimulate immune response which results in production of autoantigens. On the other hand, heteroantigens are alien substances having their origin outside the body. For instance, heteroantigens include substances produced by micro organisms like bacteria and protozoa, viruses and even substances present in snake venom, specific proteins found in foods as well as red blood cells and serum received from other individuals. Precisely speaking, an antigen is a substance that stimulates immune response. In other words, it is a substance that triggers the lymphocytes to produce antibody that directly attacks the antigen. Antigen is also called an immunogen.
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Our body is capable of producing antibodies that combat antigens or any harmful substance that enter our body or originates within the body. These antibodies work to eliminate the antigens, thereby protecting us from various ailments.
When our body comes in contact with an antigen, it immediately identifies it as an alien substance and initiates steps to combat and eliminate it. Usually our body accomplishes this task by producing antibodies, whose main job is to protect the body from invading pathogens or harmful substances that are responsible for various ailments. One can undertake several medical examinations to test these substances and ascertain whether he or she has been exposed to any harmful substance or disease-bearing pathogen.
The word "antigen" has been derived from the perception that several alien substances are capable of triggering antibody production in our immune system. These antibodies can be very helpful, for instance when the body learns to combat viruses that may cause measles or that they may be detrimental for our health by taking a cue from the allergies caused by them. It is possible to identify such exceptional traits of these antibodies by means of medical examinations. These medical tests can be helpful to find out why a particular patient is showing specific set of symptoms.
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In fact, nearly all animals possess a system which is known as the adaptive immune system. This can be described as a group of cells inside our immune system that perform specific functions, which actually help our immune system to identify as well as cope with probable threats to the host. A section of these cells become skilled at identifying substances that do not belong to the host organism. In other words, these specific cells can distinguish between the bodily substances and alien substances that may prove to be harmful for the organism. Once the antigen has been recognized, the cells in the adaptive immune system alert the other cells of the body of the problem and, subsequently, the body initiates necessary actions to combat and eliminate the alien substances.
Antigens may come from several things. For instance, humans can ingest or inhale viruses or bacteria from other organisms. Moreover, antigens may also have their origin in a toxin. In fact, a toxin is a substance which is identified by the body as an alien material and probably dangerous substance. Even transplanted tissues and organs can produce a response that leads to the generation of antibodies. This is because the body does not identify the tissues or organs as a part of the host organism. Owing to this, many people who undertake organ transplantation usually take immunosuppressive drugs that are meant to restrict the response of the immune system so that the body does not reject the transplanted organ.
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At times, our body also develops an immune response to an antigen even when something is not detrimental for our health. In such cases, it is better to call the response as an allergy. This happens when our body comes in contact with a small amount of antigen, for instance wheat, peanut butter or even a bee sting. In such cases, the helper cells in our immune system mark the substance responsible for the allergy and activate the body to generate cells that neutralize the alien substance whenever it may appear in our body in future. So when an unsuspecting person consumes peanuts, eat a slice of toast or even when he/ she is stung by a bee, the body initiates an antibody response that may result in severe uneasiness and sometimes even cause death.
There are various antigens and antigens that are most common in humans include blood factors, which help to determine the blood type of an individual. In addition, human antigens include those that aid the antibodies to slash and eliminate alien proteins that may be detrimental to our health. These are antigens that assist the body to identify the cells that belong to an individual's own system as well as the alien cells or proteins which must be eliminated from the body.
Human antigens are known as A and B. The presence or even absence of these antigens determines the type of blood one has. The blood cells in humans may contain one or both of these human antigens. Individuals having human antigen A has A type blood, while those with antigen B usually have B type blood. The blood type of people who have both the human antigens is AB. On the other hand, people who do not have either of these human antigens in their red blood cells (erythrocytes) have the blood type O. In addition, another human antigen in the blood determines whether an individual's blood type is positive or negative.
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When an individual has a particular antigen in his/ her blood, it means that they do not have the corresponding antibody in their system. On the other hand, when certain antigens are absent in the blood, it means that the individual is not capable of generating antibodies for those specific antigens. In such cases, these individuals should not be given blood transfusion as their body would not be able to produce antibodies for antigens that may be present in the donor’s blood. For example, if a person has blood type A, he/ she should never be given blood transfusion of type B blood. This is because when the body is exposed to unfamiliar blood antigens it induces an immune response that may result in grave complications or even lead to the patient's death.
The human leukocyte (white blood cell) antigen system creates a human antigen, which facilitated identification of antigens belonging to alien proteins. In fact, the 6th chromosome of our DNA (deoxyribonucleic acid) contains the code of the human leukocyte antigen system. This particular part of the DNA produces proteins that identify antigens. The process by which antigens are identified by the specific immune cells is known as antigen presentation.
Nearly all the antigens found in the human body are those that have got into the body via the bloodstream, from the air or digestive tract. These antigens are also known as exogenous antigens and the human immune system considers these to be harmful. There is another type of antigen present in the human body and it is known as endogenous antigen. Generally, the endogenous antigens are alien substances that enter the human body and overwhelm the healthy cells and reproduce inside the healthy cells. The process antigen presentation identifies such foreign substances and helps the body to destroy them by triggering an immune response which generates antibodies.
There are a number of types of antigens and they include complete antigen or immunogen, incomplete antigen or hapten, alloantigens, autoantigens, heterophilic or cross-reacting antigens. Below is a brief discussion on each type of antigens.
Complete antigen or immunogen: This type of antigen is basically molecules (which include proteins or polysaccharides) which are able to trigger an immune response all by themselves. Their molecular weight is very high (above 10,000) and some examples of complete antigens include bacteria, viruses, fungi and parasites.
Incomplete antigen or hapten: This type of antigen generally comprises non-protein substance. Incomplete antigens cannot induce immune response by themselves and have low molecular weight (below 10,000). Therefore, they need a carrier molecule to transform them into immunogenic. Serum protein like globulin or albumin is non-antigenic components, but they work as carrier molecules helping the incomplete antigen to trigger immune response. Simple haptens are relatively small molecules and do not form visible precipitate with antibodies. Some examples of haptens include glycoproteins, polysaccharide "C" of beta-haemolytic Streptococci. On the other hand, complex haptens such as capsular polysaccharide of cardiolipin and Pneumococci are comparatively larger molecules and are able to form visible precipitate by coming together with specific antibodies.
Alloantigens: This type of antigen includes the body’s compounds or cells and also the antigenic products which are processed by macrophages. Later, the alloantigens are taken up by the cytotoxic T-cells. Interestingly, this type of antigen is present in a particular individual, but absent in other individuals belonging to the same species. Some examples of alloantigens include the blood group antigens (which comprise A and B on the surface of the red blood cells), HLA (Histocompatibility leukocyte antigens) and others.
Autoantigens: Generally, an autoantigen is a normal protein or complex of proteins. Sometimes, it also comprises DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). When we talk about autoantigens we denote proteins such as sperm protein, lens protein, myelin basic protein, thyroglobulin, DNA and others which are sequestered from an individual’s own blood circulation. When they are injected in the blood, they trigger an immune response. The immune system identify autoantigens are identified as antigens in people who may be enduring any particular autoimmune disease. Under normal circumstances, these antigens are usually not targeted by the immune system. However, at times, the normal immunological tolerance for these antigens is lost in some patients owing to genetic as well as environmental factors.
Heterophilic or cross-reacting antigens: When an antibody generated against a particular antigen binds against a different antigen, it is known as heterophilic or cross-reacting antigen. Although such antigens have the same nature, they are never identical. These antigens can be present in different tissues of several species. Some examples of heterophilic or cross-reacting antigens include the antibody generated against Rickettsia, which binds some Proteus species and the antibody generated agains M-protein of S pyogens that binds with the heart muscles in humans.
