Training Your Immune System

A healthy immune system helps protect you from getting sick by identifying and destroying intruders in your body, like bacteria and viruses. Vaccines teach your body how to fight these intruders so that when they get into your body, your immune system can fight them without making you sick.

“Vaccines are like a training course for the immune system,” says Northwestern Medicine Allergist and Immunologist Deeba Masood, MD. “They prepare the body to fight disease without exposing it to disease symptoms.”

Vaccines prevent an estimated 2 to 3 million deaths per year.

The U.S. vaccine safety system has protocol in place to make sure that all vaccines are as safe as possible.

Unlike many vaccines that require an injection with a weakened or inactivated virus, two COVID-19 vaccines use a new technology: messenger ribonucleic acid (mRNA). When injected, mRNA vaccines teach your cells to make a piece of a protein that is found on the surface of the virus that causes COVID-19.

• When this protein appears on the surface of a cell, your immune system does not recognize it.
• Your system then begins building an immune response by making antibodies to destroy the intruder.
• This process is the same as what would happen if you were infected with the virus itself.
• Once the “training” is complete, your immune system will know how to fight the virus that causes COVID-19 if you are exposed to it in the future.

Once your cells make the protein, they destroy the mRNA instructions. It is important to note that these vaccines do not affect your cells’ DNA.

Here’s how vaccines— including the new mRNA vaccines— work.


COVID-19 vaccines are supplied by state and local agencies. Not all vaccine types are offered at all Northwestern Medicine locations. Please visit the COVID-19 Public Vaccine Distribution section on the COVID-19 Resource Center for more information.