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An antigen is a substance that causes an immune response in the body—specifically, an antigen causes the body to produce antibodies. Antigens are things like viruses, bacteria, toxins, cancer cells, and other foreign substances, like the cells of a transplanted organ.

An antibody is a protein produced by some cells as part of the immune system’s defenses. Antibodies attach to antigens and make them harmless or help the body to destroy them.

Antigens are studied in fields like immunology, virology, and pharmacology. Most vaccines contain forms of antigens that trigger the body to produce specific antibodies to fight that antigen.

There are a lot of things that can get inside the body that we don’t want to be there, like viruses, bacteria, and toxins—all of these things are antigens. But antigen doesn’t necessarily mean something bad—it specifically refers to a substance that causes the body to create antibodies. (For example, the cells of transplanted organs can be considered antigens if they trigger an immune response.) In fact, antigen is a combination of the word antibody and the suffix –gen, which means “that which produces.”

When antigens enter the body, a healthy immune system goes on alert. White blood cells called lymphocytes identify and target the antigen and start the production of antibodies. The antibodies find the antigens, attach to them, and then deactivate them or lead them to special white blood cells (called macrophages) that basically eat and destroy them (cue the Pac-Man music). If the same antigen shows up again, the body will be able to quickly manufacture more of those specific antibodies to counter the threat.

This is how vaccines work. Vaccines consist of a small amount of an antigen, such as a virus or bacterium, that has been killed, weakened, or modified so that it’s not harmful. Introducing this small amount trains the body to identify that specific antigen and create defenses against it in case it ever appears in the body again.

Sometimes, antigens are identified as harmful by our bodies when they’re really not. This can happen with transplanted organs and in autoimmune diseases, but also in a much more common scenario: allergies. An allergen like pollen acts as an antigen, causing the immune system to kick in when it doesn’t really need to.

Blood types (A, B, AB, and O) are based on the presence or absence of specific antigens on red blood cells.

This graphic shows an illustration of antibodies attacking an antigen—a virus.

What makes this program unique is that four vaccine candidates are being tested simultaneously, each representing a unique combination of mRNA format and antigen. The design of the trial allows us to move urgently while preserving the highest quality and safety standards.

— AlbertBourla (@AlbertBourla)

A human monoclonal antibody blocking SARS-CoV-2 infection

This antibody could be useful for development of antigen detection tests and serological assays targeting SARS-CoV-2, and ultimately may offer a therapeutic option against COVID-19.

— Ilan Schwartz MD PhD (@GermHunterMD)

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• antibody
• immune system
• vaccine
• pathogen
• virus
• bacteria
• disease
• cancer
• toxin
• immunology
• pharmacology

What do antigens trigger the production of?��

A. viruses
B. allergies
C. pathogens
D. antibodies