Vaccination exploits the adaptive immune system's capacity for immunological memory. Vaccines expose the immune system to antigens in a controlled manner, priming both B-cell (humoral) and T-cell (cellular) responses without causing disease. Upon antigen presentation by dendritic cells to naive T cells in lymph nodes, CD4+ T-helper cells differentiate into effector and memory subsets. T-follicular helper (Tfh) cells drive germinal center reactions where B cells undergo somatic hypermutation and affinity maturation, producing high-affinity antibodies. Long-lived plasma cells migrate to bone marrow and produce sustained antibody titers. Memory B cells and memory T cells persist for years to decades, enabling rapid anamnestic (secondary) response upon re-exposure. Vaccine types differ in their immunogenic mechanisms: (1) Live attenuated vaccines (MMR, varicella, rotavirus, LAIV, yellow fever) contain weakened pathogens that replicate limitedly, producing robust humoral AND cellular immunity with typically lifelong protection after 1-2 doses; (2) Inactivated/killed vaccines (IPV, hepatitis A, inactivated influenza) cannot replicate and primarily stimulate humoral immunity, often requiring multiple doses and boosters; (3) Subunit/conjugate vaccines (HBV recombinant, HPV, PCV13, MCV4) contain purified antigens — conjugation of polysaccharide antigens to carrier...