Study Overview: This study looks at how vaccines work against S. pneumoniae, a germ causing illnesses like pneumonia. Healthy adults (ages 20-40) will be given a vaccine named Prevenar13 to see how their immune systems respond. The focus is on germinal center B cells, a type of white blood cell important for fighting infections. To study this, a small needle will be used to collect cells from lymph nodes, which are small glands that help fight infections.

Study Procedures: Participants will receive one vaccination. Blood samples and lymph node samples will be collected at different times over three months. An ultrasound will check the lymph nodes' size regularly. The study requires eight visits for sample collection.

Eligibility: Participants must be healthy adults aged 20-40 years. People with certain health conditions or who recently had surgery, are pregnant or breastfeeding, or have received recent vaccinations cannot join.

• The study lasts about three months with eight visits.
• Participants will be compensated for their time.
• Understanding immune response could help make better vaccines in the future.