How do vaccines train the immune system?

Vaccines imitate an infection without causing the disease. They present the immune system with an antigen, a substance that prompts the body to produce antibodies. The immune system responds and builds a memory of the germ, so that if the real pathogen arrives later, the response is faster and stronger. As the CDC puts it, vaccines work by imitating an infection to engage the body's natural defenses. [1] [2]

A pathogen is a bacterium, virus, parasite or fungus that can cause disease. Each is made of subparts, and the one that triggers antibody formation is called an antigen. Antibodies are proteins made by white blood cells, and each is trained to recognize one specific antigen. The World Health Organization compares them to soldiers in the body's defense system. [3] [1]

The first time the body meets an antigen, it takes time to make specific antibodies, and during that gap the person can fall ill. Once the antibodies exist, they work with the rest of the immune system to destroy the pathogen. The body also creates memory cells that stay alive after the pathogen is defeated. On a later exposure, these cells let the body produce antibodies much faster and more effectively. [3] [4]

The active ingredient is an antigen. It may be a weakened or killed bacterium or virus, a piece of its surface or genetic material, or a bacterial toxin treated to make it non-toxic. The European vaccination information site lists six types: mRNA, viral-vector, inactivated, live attenuated, toxoid and subunit vaccines. [1] [5]

Newer vaccines carry the blueprint for making an antigen instead of the antigen itself. An mRNA vaccine delivers instructions that lead cells to make a harmless protein matching part of the virus. A viral-vector vaccine uses a harmless virus to deliver a small section of a disease-causing virus's genetic code. In both cases the immune system recognizes the protein as foreign and produces antibodies. Because these vaccines supply only a small piece of the virus, they cannot cause the illness. [5] [3]

Some vaccines need several doses weeks or months apart, which the WHO says can help produce long-lived antibodies and memory cells. Live attenuated vaccines give a strong, long-lasting response, so fewer doses may be needed. [3] [5]

Immunity can take weeks to develop, so infection is possible shortly after vaccination. Vaccinated people can also sometimes become infected, but they are far less likely to die or become seriously ill than someone whose immune system is unprepared. Antibodies to one pathogen generally do not protect against another, unless the two are very similar. [1] [3]