Vaccination & Memory

How vaccines harness immunological memory to provide protection without disease — one of the greatest achievements in medical history.

Mechanism

How Vaccines Work

Vaccines introduce a harmless form of a pathogen (or its components) to train the adaptive immune system, creating memory cells without causing disease.

1. Antigen Introduction

The vaccine delivers antigens (weakened pathogen, protein subunit, or mRNA instructions) to the body, mimicking a natural infection.

2. Immune Activation

Dendritic cells capture antigens and present them to T and B cells in lymph nodes. Helper T cells coordinate the response, B cells produce antibodies.

3. Memory Formation

After clearing the antigen, long-lived memory T and B cells persist. Upon re-exposure to the real pathogen, they mount a rapid, powerful response.

Immune Memory

Primary vs. Secondary Response

Primary Response

Response time: 7–14 days

Antibody Level15%

First exposure to antigen. Slow antibody production, mainly IgM. Naive B and T cells must be activated and undergo clonal expansion.

Secondary Response

Response time: 1–3 days

Antibody Level85%

Re-exposure to same antigen. Rapid, massive antibody production, mainly IgG. Memory cells are quickly reactivated.

Types

Types of Vaccines

Different vaccine technologies use different strategies to present antigens to the immune system.

Type	Examples	Mechanism	Pros	Cons
Live Attenuated	MMR, Varicella, Yellow Fever	Contains weakened (attenuated) live pathogen that replicates but cannot cause disease in healthy individuals.	Strong, long-lasting immunity; often single dose sufficient	Not suitable for immunocompromised; requires cold storage
Inactivated	Influenza (shot), Hepatitis A, Polio (IPV)	Contains killed pathogen that cannot replicate. Stimulates immune response without infection risk.	Safe for immunocompromised; stable storage	Weaker immune response; often requires boosters
Subunit / Recombinant	Hepatitis B, HPV, Pertussis (acellular)	Contains purified proteins or polysaccharides from the pathogen surface, not the whole organism.	Very safe; targeted immune response	May need adjuvants; multiple doses often required
mRNA	COVID-19 (Pfizer, Moderna)	Delivers mRNA encoding a pathogen protein. Host cells produce the protein, triggering immune response.	Rapid development; strong humoral and cellular response	Requires ultra-cold storage; relatively new technology
Viral Vector	COVID-19 (AstraZeneca, J&J), Ebola	Uses a harmless virus to deliver pathogen genetic material into cells, which then produce the target antigen.	Strong immune response; single dose possible	Pre-existing immunity to vector may reduce efficacy
Toxoid	Tetanus, Diphtheria	Contains inactivated toxins (toxoids) produced by the pathogen, training the immune system to neutralize the toxin.	Targets the disease mechanism directly	Only effective against toxin-producing pathogens; needs boosters