Antigens are molecules that initiate the immune response and can be bound by antibodies. Antibodies are proteins produced by B-lymphocytes that bind to specific antigens.

In immunology, an antigen is a substance that evokes an immune response. Formally they are defined as a substance that causes the production of antibodies specific to that antigen; however, they also cause T cell-mediated immune responses, and may lead to an inflammatory response. The substance may be from the external environment or formed within the body. The immune system will try to destroy or neutralize any antigen that is recognized as a foreign and potentially harmful invader.”Self” antigens are usually tolerated by the immune system; whereas “non-self” antigens can be identified as invaders and can be attacked by the immune system.

Antigens have several structural components of interaction that may be bound by different classes of antibodies. Each of these distinct structural components is considered to be an epitope, also called an antigenic determinant. Therefore, most antigens have the potential to be bound by several distinct antibodies, each of which is specific to a particular epitope. The antigen-binding receptor on an antibody is called a paratope and is specific to the epitope of the antigen. Using the “lock and key” metaphor, the antigen itself can be seen as a string of keys – any epitope being a “key” – each of which can match a different lock.

These antibodies circulate in the bloodstream and lymphatic system, binding with the antigen whenever it is encountered. The binding can fight infection in several ways. Antibodies can bind to viruses or bacteria, which interferes with the chemical interactions required for them to infect or bind to other cells. The antibodies may create bridges between different particles containing antigenic sites, clumping them all together and preventing their proper functioning. Antibody neutralization can prevent pathogens from entering and infecting host cells. The neutralized antibody-coated pathogens can then be filtered by the spleen to be eliminated in urine or faeces. The antigen-antibody complex stimulates the complement system described previously, destroying the cell bearing the antigen. Antibodies also opsonize pathogen cells, wherein they mark them for destruction by phagocytic cells, such as macrophages or neutrophils. Additionally, antibodies stimulate inflammation, while their presence in mucus and on the skin prevents pathogen attack.

 

Practice Questions

MCAT Official Prep (AAMC)

Biology Question Pack, Vol. 1 Passage 2 Question 6

Practice Exam 4 B/B Section Passage 5 Question 26

 

Key Points

• An antigen is a molecule that initiates the production of an antibody and causes an immune response.

• Antigens are typically proteins, peptides, or polysaccharides. Lipids and nucleic acids can combine with those molecules to form more complex antigens, like lipopolysaccharide, a potent bacterial toxin.

• An epitope is a molecular surface feature of an antigen that can be bound by an antibody. A paratope is the molecular surface feature of an antibody that binds to an epitope.

• Antibodies fight infections in three ways: they mark pathogens for destruction by phagocytic cells in a process known as opsonization, they coat key sites on pathogens necessary for infection, and they induce the complement cascade to occur against antibody-bound pathogens.

Key Terms

Antigen: A substance that induces an immune response, usually foreign, but self-antigens and internally produced antigens exist as well.

Antibody: a protein produced by B-lymphocytes that binds to a specific antigen

Opsonize: to make (bacteria or other cells) more susceptible to the action of phagocytes by use of opsonins

Self-antigens: antigens in the body of an individual

Non-self antigens: antigens from outside the body of an individual

T cell: a lymphocyte, from the thymus, that can recognize specific antigens and can activate or deactivate other immune cells

Epitope: the part of an antigen molecule to which an antibody attaches itself

Paratope: a part of an antibody which recognizes and binds to an antigen

Phagocytic: cells that protect the body by ingesting harmful foreign particles