The most common human antigens are the blood factors, which determine a person’s blood type. Human antigens also include those antigens that help antibodies cleave to and destroy foreign proteins that can cause harm to a person’s body. These antigens help the body to distinguish cells that belong to a person’s own system from foreign cells or proteins that that need to be removed.
Human blood cells may contain one or both of two human antigens. These human antigens are called A and B, and their presence or absence determines what blood type a person has. People with blood type A have the human antigen A, and those with blood type B, have human antigen B. An individual with type AB blood has both, and people with type O blood do not have either of these antigens on their red blood cells. There is also a human blood antigen that determines whether a person has a positive or a negative blood type.
The presence of a blood type antigen means that the corresponding antibody is absent from a person’s system. Conversely, the absence of certain antigens means that the person does create antibodies for those particular antigens, which is why blood transfusions must not be given if they contain a human antigen not normally present in the patient’s body. For instance, someone with blood type a should not be given a transfusion of type B blood. Unfamiliar blood antigens elicit an immune response, which can lead to serious complications or even the death of the patient.
A human antigen is created by the human leukocyte antigen system, which helps identify antigens that belong to foreign proteins. The human leukocyte antigen system is coded on human deoxyribonucleic acid (DNA) on the 6th chromosome. This section of DNA is used to create proteins that recognize antigens. The process of identifying an antigen is called antigen presentation.
Most antigens in the human body are those that have entered the body through the air, bloodstream, or digestive tract. These are called exogenous antigens and are considered hostile by the human immune system. The other types of antigens that exist within a person’s body are called endogenous antigens. They are often foreign invaders that have taken over a healthy human cell and replicated inside of it. Antigen presentation allows these invaders to be identified and destroyed by the antibodies in the immune system.