Vaccination — Introducing the Weakened Virus
A live but attenuated virus enters the body harmlessly.
- 10⁵–10⁶: Viral load administered (plaque-forming units per dose)
- 20–200: Attenuation passages (cell-culture generations weakened)
- <0.01%: Disease risk (near-zero pathogenicity)
- 3–14 days: Replication window (limited local viral replication)
Attenuation reduces virulence
Serial passage weakens the virus without destroying its antigens.
Attenuated strains still replicate briefly, mimicking natural infection safely.
Injection site drainage
Lymphatic vessels carry virus particles toward the nearest lymph node.
Why live vaccines work
Live virus triggers broader, longer-lasting immunity than inactivated vaccines.
Innate Response — Dendritic Cells Engage
Dendritic cells capture viral particles and process their antigens.
- 50–200: Dendritic cells activated (per lymph node region)
- ~6 hrs: Antigen processing time (capture to surface display)
- IFN-α, IL-12: Cytokines released (innate alarm signals)
- Class I & II: MHC presentation (antigen shown on surface)
Sentinel cells patrol tissue
Dendritic cells constantly sample surrounding tissue for foreign particles.
Antigen processing and display
Captured viral proteins are broken down and shown on MHC molecules.
MHC presentation bridges innate detection and adaptive immune activation.
Migration to lymph node
Activated dendritic cells travel to lymph nodes carrying antigen.
Adaptive Activation — Naive Cells Respond
Naive B-cells and T-cells recognize antigen and become activated.
- Hundreds: Naive B-cells engaged (antigen-specific clones selected)
- Hundreds: Naive T-cells engaged (helper and killer subsets)
- 2 signals: Activation threshold (antigen plus costimulation)
- 1–3 days: Time to activation (after antigen presentation)
Clonal selection begins
Only lymphocytes matching the viral antigen shape get activated.
T-cell help for B-cells
Helper T-cells license B-cells to mature and multiply.
Two-signal activation prevents accidental attacks on healthy tissue.
Germinal centers form
Activated B-cells cluster inside lymph nodes to refine antibodies.
Clonal Expansion — Lymphocytes Multiply
Antigen-specific cells divide rapidly, building a large defense force.
- 1,000–50,000×: Expansion factor (per activated clone)
- ~6 hrs: Division time (per lymphocyte cycle)
- Day 7–10: Peak cell count (post vaccination)
- 10–100×: Antibody titer rise (above baseline)
Exponential division
Each activated lymphocyte divides repeatedly within days.
Affinity maturation
Germinal center B-cells mutate to bind antigen more tightly.
Somatic hypermutation sharpens antibody precision with every division cycle.
Effector cell output
Plasma cells pour antibodies into blood and lymph.
Memory Formation — Cells That Persist
A small fraction of lymphocytes become long-lived memory cells.
- ~5–10%: Memory B-cells surviving (of peak expansion)
- ~5–10%: Memory T-cells surviving (of peak expansion)
- Decades: Persistence duration (measured in years)
- Hours–days: Recall response speed (vs weeks for naive cells)
Contraction phase
Most effector cells die off after the infection clears.
Memory cells persist
Surviving cells settle into bone marrow and lymphoid tissue.
Memory cells can outlast the original vaccination by many decades.
Rapid recall on exposure
Memory cells react faster and stronger than naive cells.