The science behind teaching your body to recognize and fight pathogens
Vaccines work by mimicking infection without causing disease. They teach your immune system to recognize specific pathogens, so when you encounter the real thing, your body can respond quickly and effectively.
This "training" happens because vaccines contain antigens — molecules that trigger an immune response. These antigens might be weakened or killed pathogens, pieces of the pathogen (like proteins or polysaccharides), or genetic instructions for your cells to make these pieces.
The key is that vaccines stimulate the adaptive immune system to create memory cells without causing the symptoms and complications of actual infection.
Modern vaccines use different approaches to train the immune system. Each type has advantages for different situations:
Contain weakened but living versions of the virus or bacteria. These closely mimic natural infection and typically provide strong, long-lasting immunity with one or two doses. Examples: measles-mumps-rubella (MMR), yellow fever, oral polio vaccine.
Contain killed pathogens that can no longer replicate. These are very safe and stable but may require multiple doses or boosters for sustained protection. Examples: inactivated polio vaccine, hepatitis A, rabies vaccine.
Contain only essential pieces of the pathogen — proteins, polysaccharides, or sugar coatings (antigens) — not the whole organism. These are highly safe and well-tolerated. Examples: tetanus toxoid, hepatitis B, pneumococcal polysaccharide vaccine.
Contain messenger RNA that instructs cells to produce a harmless piece of the pathogen (typically a spike protein). Your immune system then recognizes this protein. The mRNA is quickly broken down and does not enter the nucleus or alter DNA. Examples: Pfizer-BioNTech and Moderna COVID-19 vaccines.
Use a harmless virus (the vector) to deliver genetic instructions for making a pathogen protein. The immune system learns to recognize this protein. Examples: Johnson & Johnson COVID-19 vaccine, Ebola vaccine.
When the vaccine enters your body, antigen-presenting cells (APCs) — primarily dendritic cells — capture the vaccine antigens and display them on their surface. This is the same process that happens with real pathogens.
Helper T cells recognize the antigen fragments displayed by APCs and become activated. These helper T cells coordinate the immune response by releasing signaling molecules (cytokines).
With guidance from helper T cells, B cells specific to the antigen become activated and multiply. These B cells differentiate into plasma cells that produce large quantities of antibodies tailored to the specific antigen.
Most plasma cells die off after the infection is cleared, but some B cells and T cells become long-lived memory cells. These memory cells persist in your body, sometimes for decades, ready to respond quickly if you encounter the same pathogen again.
If you're exposed to the actual pathogen, memory B cells quickly produce antibodies while memory T cells rapidly multiply and attack infected cells. This response is often fast enough to prevent the pathogen from causing disease — or at least from causing severe disease.
Memory cells can eventually lose their "memory" or their numbers can decline over time. Additionally, some pathogens (like influenza) evolve rapidly, producing new variants that our immune system doesn't recognize as well.
Boosters refresh and enhance immune memory by exposing the immune system to the antigen again. This causes:
The timing and number of booster doses are carefully designed based on research into how long protection lasts for each specific vaccine.
• CDC. "Understanding How Vaccines Work." Centers for Disease Control and Prevention.
• WHO. "How Do Vaccines Work?" World Health Organization, 2020.
• Pollard AJ, Bijker EM. A guide to vaccination: Immunogenicity, clinical trials and age-groups. Lancet. 2021;397(10270):302-312.