Innate & adaptive immunity — pathogens, antibodies, clonal selection, memory cells
Clonal selection: B cells with receptors matching an antigen are selected, proliferate, and differentiate into plasma cells (antibody factories) and long-lived memory cells. Memory cells enable rapid response on re-exposure — the basis of vaccination.
The adaptive immune response takes 7–14 days on first exposure (primary response) but only 1–3 days on re-exposure thanks to immunological memory (secondary response). Vaccines prime memory without causing disease, reducing the primary response to a secondary one when real pathogens appear.