How the Body Defends Against Viruses
Understanding how the body’s immune system defends against disease helps explain how vaccines—especially COVID-19 vaccines—work.
When a virus such as SARS-CoV-2, the virus that causes COVID-19, enters the body, it begins to multiply. This invasion, known as an infection, leads to the development of disease symptoms.
The immune system uses several different tools and mechanisms to fight infection. The following is a general overview:
Blood contains red blood cells, which transport oxygen to the body’s tissues and organs, and white blood cells (immune cells), which are responsible for fighting infections.
Different types of white blood cells combat infections in different ways:
Macrophages
Macrophages are white blood cells that engulf and digest germs, as well as dead or dying cells. They identify parts of invading microorganisms called antigens. Once the body recognizes these antigens as foreign, it is stimulated to produce antibodies that attack them.
B Lymphocytes (B Cells)
B lymphocytes are immune cells that produce antibodies, which specifically recognize and attack antigens.
T Lymphocytes (T Cells)
T lymphocytes are another type of immune cell that plays an essential role in protecting the body against infections.
When a person is infected with COVID-19 for the first time, it may take several days or even weeks for the immune system to produce and activate the necessary defenses to fight the infection.
After recovery, the immune system remembers how to protect the body against that virus if it is encountered again. It retains a small number of memory T cells (memory T lymphocytes), which enable a rapid immune response upon re-exposure to the same virus. When the familiar virus enters the body again, B lymphocytes quickly begin producing antibodies to eliminate it.
Researchers continue to study how long these memory cells remain active after COVID-19 infection and how long they provide protection against future infections.
How COVID-19 Vaccines Work
COVID-19 vaccines help develop and strengthen the body’s immune system against the virus without causing COVID-19 disease.
Although different vaccines work in different ways, they all stimulate the body to produce memory cells (both T lymphocytes and B lymphocytes) that “remember” how to recognize and fight the virus if the body is exposed to it again in the future.
Types of COVID-19 Vaccines and How They Work
In general, there are three main types of COVID-19 vaccines that have either been approved by international health organizations or have undergone large-scale clinical testing. Below is a brief explanation of each type and how it protects against COVID-19.
mRNA Vaccines
mRNA vaccines contain a small piece of the virus’s genetic material. This genetic code instructs the body’s cells to produce a unique protein found on the coronavirus.
Once the cells have made copies of this protein, the vaccine’s genetic material is broken down and removed. The immune system recognizes that the viral protein does not belong in the body and responds by producing T lymphocytes and B lymphocytes. As a result, if the body encounters the real virus in the future, the immune system is prepared to fight it quickly.
Protein-Based Vaccines
Protein-based vaccines contain a portion of a viral protein that causes COVID-19.
After vaccination, the immune system recognizes that this protein is foreign and responds by producing T lymphocytes and antibodies. These immune responses create memory that enables the body to recognize and fight the virus more effectively if it is encountered again.
Viral Vector Vaccines
Viral vector vaccines use a modified virus that is different from the virus responsible for COVID-19. Inside the outer shell of this harmless modified virus is a piece of the genetic material from the SARS-CoV-2 virus. This modified virus is known as a viral vector.
Once the viral vector enters the body’s cells, it delivers genetic instructions that tell the cells to produce a protein unique to the COVID-19 virus. The cells then make copies of this viral protein, prompting the immune system to generate B lymphocytes and T lymphocytes. These immune cells help protect the body if it is exposed to the virus in the future.
