Vaccines stimulate adaptive immunity by presenting antigens (inactivated organisms, live-attenuated organisms, toxoids, subunit proteins, or mRNA) to antigen-presenting cells, activating both humoral (B-cell antibody production) and cell-mediated (T-cell) immune responses. Primary vaccination produces IgM followed by class-switching to IgG, while booster doses stimulate memory B cells to mount a rapid, high-affinity IgG anamnestic response. Live vaccines (MMR, varicella, rotavirus) produce stronger and longer-lasting immunity but are contraindicated in immunocompromised patients. Conjugate vaccines (PCV13, Hib, MenACWY) link polysaccharide antigens to protein carriers, converting T-cell-independent responses to T-cell-dependent responses, enabling effective immunization in children under 2 years whose immune systems cannot mount adequate responses to polysaccharide antigens alone.