How Do Vaccines Work?

By Gaia Yun





How do Viruses spread?

(virus is not to scale)

-Viruses enter the body and attach onto the cell surface.

-Virus antigens (unique molecular markers on virus surface) bind to cell surface receptors.

-The cell receptors mistakenly "recognize" the viruses via the antigens, and the cell lets the virus in through the membrane in a variety of ways.

-Viruses insert their own genomes into the cell DNA, turning the cell into a viral protein factory. The cell cannot effectively continue its own functions.

-Proteins are assembled, and new viruses break out of the destroyed cell. They search for new cells in order to replicate themselves.

What Are Antibodies?

-Antibodies are Y-shaped proteins assembled by B plasma cells.

-There are hundreds of types of B-cells that each create a specific antibody.

-Upon receiving a signal, naive B cells develop into B Plasma cells that produce antibodies.

-Antibodies have a part that binds to the antigen of a pathogen (yellow highlight), as well as a part that binds to cells(green highlight).

-Antibodies bind to the antigens on a virus, preventing the virus from binding to the cell surface.

-They work as opsonins - they help phagocytes identify the pathogen and absorb it.

Antibodies also:

-activate the classical pathway of complement system, which destroys pathogen

-attach to and neutralize toxins

-cause agglutination: clump together on pathogen surface to slow pathogen down

-allow T killer cells to identify and destroy cells infected with pathogen

The Body's Adaptive Immune Response to a Virus

The above model shows how the adaptive immune system reacts against pathogens. It' s important to note that the innate immune system(orange) is also working to reduce pathogens. The innate system launches a non-specific, automatic response against intruders, and exists at the location of viral infection. In the innate system:

-macrophages and neutrophils use phagocytosis to destroy pathogen

-macrophages release cytokines(signalling proteins) and induces inflammation, recruiting more phagocytes

-inflammation stimulates secretion of interleukins into brain, causing fever and lethargy

The dendritic cells are the link between the innate and adaptive immune systems. They absorb and present antigens (purple), then move through the lymph nodes searching for naive T and B cells that can create the specific corresponding antibody. Thus, the adaptive immune system is activated:

-Cellular immunity(green): T killer cells destroy infected cells. T helper cells produce cytokines that help trigger the humoral immune system.

-Humoral immunity(blue): B plasma cells create antibodies.

The process also produces memory B cells, memory T helper cells, and memory T killer cells, which remember the specific pathogen and react quickly if it enters the body again. The created antibodies remain in the body, also ready to mobilize against future invasion. After initial exposure, the adaptive immune system as a whole becomes efficient at recognizing and destroying the pathogen.

Immune Response: Virus V.S. Vaccine

Immune Response to a Virus:

Before the adaptive system kicks in, the patient suffers from fever, lethargy, and other symptoms caused by the innate system. During this time, cell damage may also occur. As antibody count increases, the patient recovers. The second time the pathogen enters the body, antibody concentration swiftly increases as the adaptive system responds, preventing the patient from getting sick.

Immune Response to a Vaccine:

Vaccines present viral antigens. However, they do not cause sickness, because vaccines don't actually invade and destroy cells. Still, the presence of antigens prompts a response from the adaptive immune system; memory cells and antibodies are created. During "secondary exposure," when the virus actually invades, the immune system knows what to do.

Types of Vaccines: What Exactly is a Vaccine?

Live attenuated vaccines are a weakened form of the virus. They are created by repeated culturing in labs. Because these are so similar to the original virus, they cause a virtually identical adaptive immune response, and only require 1-2 doses for lifetime protection against the specific strain. The vaccine has limitations because it cannot be administered to immunodeficient people , and though it's very unlikely, it has the potential to mutate and cause disease (however, the only vaccine known to do this is the oral polio vaccine.)

Used for: measles, mumps, rubella, vaccinia, varicella, zoster, yellow fever, rotavirus, intranasal influenza, oral polio

Inactivated vaccines are viruses "killed" by heat or chemicals(formalin); its DNA is destroyed. The vaccine can never replicate and cause disease, even in immunodeficient people. However, this vaccine requires multiple doses to elicit a protective immune response. While live attenuated vaccines stimulate cellular and humoral immunity, inactivated vaccines only cause humoral immunity. Additionally, antibodies corresponding to inactivated vaccines eventually diminish, so vaccines must be re-administered.

Used for: polio, hepatitis A, rabies, influenza

Subunit vaccines only include certain parts of the virus, like its antigens or proteins. Conjugate vaccines are assembled out of two or more such parts. Like whole inactivated vaccines, these are substantially weaker at eliciting a response than live attenuated vaccines. Subunit vaccines can also be created through genetic engineering; these are called recombinant vaccines.

Used for: hepatitis B, influenza, human papillomavirus, anthrax


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The content of these pages was created by students for students with the help of educators and scientists. The views expressed herein are those of the authors and do not necessarily reflect the views of NSF or ISB.