The immune system learns. Encounter a pathogen, survive it, and the body retains a memory that responds faster next time. Vaccines create that memory without the illness.
The mechanism
The immune system recognises specific molecular shapes on a pathogen – antigens. When it encounters one, it produces antibodies that bind to that shape and cells that remember it.
A vaccine presents the antigen without the disease. The memory forms either way.
The types
Live attenuated – a weakened version of the pathogen that replicates poorly. Produces strong, long-lasting immunity. Not suitable for people with compromised immune systems.
Inactivated – the pathogen killed. Safe, generally requires boosters.
Subunit – just the antigen, not the whole organism. Very safe, often needs an adjuvant to provoke a sufficient response.
mRNA – instructions for your own cells to make the antigen briefly. The cells produce it, the immune system responds, the instructions degrade within days.
Viral vector – a harmless virus modified to carry the instructions.
Why some need boosters
Immune memory fades at different rates for different pathogens. Some vaccines produce lifelong protection; others decline over years. That is a property of the immune response, not a flaw in the vaccine.
What immunity means
Usually reduced probability and severity rather than absolute prevention. A vaccine that prevents most infections and makes the rest mild is doing its job.
The population effect
When enough people are immune, transmission chains break, which protects those who cannot be vaccinated. That is the mechanism behind eradication programmes.