Anti-Wha?

What is an Antigen?

An antigen is, quite simply, a molecule that stimulates the generation of antibodies. Hence, an antigen is usually a molecule found on a foreign substance, whether it is a pathogen or not, and the immune system will generate antibodies to destroy it.

In the human immune system, an antigen basically helps identify the organism it belongs to as being foreign, and something that needs to be destroyed by leukocytes.

Although it was named because it was thought to stimulate the generation of antibodies, the modern usage of the word includes everything that the human immune system can recognize as belonging to a pathogen, and anything that stimulates the immune response itself, and not only the generation of antibodies.

What is an Antibody?

An antibody, or immunoglobulin, is a Y-shaped protein that is the body’s response to an antigen. The antibody is antigen-specific, and can only attach itself to a specific antigen. This is because the two “tips” of the Y-shape end in paratopes that bind to specific epitopes on the antigen. This is very analogous to the lock-and-key concept, or to an enzyme-and-substrate concept.

This lock-and-key method allows the antibody to either tag the antigen so that leukocytes may come and destroy it, or to inhibit its vital functions on its own. Antibodies are attached to B-cells unless released by the B-cell in order to take action against a specific pathogen invader.

See Figure 1.K to see how antibodies and antigens bind.

Figure 1.K - The binding of an antigen to an antibody.