The immune system comprises innate (nonspecific, immediate) and adaptive (specific, memory-forming) immunity. Innate immunity includes physical barriers (skin, mucous membranes), cellular defenses (neutrophils, macrophages, NK cells), and complement system activation. Adaptive immunity involves humoral (B-cell/antibody-mediated) and cell-mediated (T-cell) responses. B cells differentiate into plasma cells producing antibodies (IgG, IgM, IgA, IgE, IgD) and memory B cells. T cells include CD4+ helper T cells (coordinate immune response), CD8+ cytotoxic T cells (kill infected cells), and regulatory T cells (prevent autoimmunity). Vaccines exploit adaptive immunity by presenting antigens that stimulate the immune response without causing disease. Live attenuated vaccines (MMR, varicella, rotavirus, intranasal influenza) contain weakened organisms that replicate and produce broad, long-lasting immunity. Inactivated/killed vaccines (influenza injection, hepatitis A, rabies) contain non-replicating organisms requiring multiple doses and boosters. Subunit/toxoid vaccines (hepatitis B, DTaP, HPV, pneumococcal conjugate) contain purified antigens or inactivated toxins. mRNA vaccines (COVID-19 Pfizer/Moderna) instruct cells to produce spike protein antigen, triggering both humoral and cellular immune responses. Primary immune response (first exposure) produces IgM first, then IgG over 1-2 weeks. Secondary response (booster/re-exposure) produces rapid, high-titer IgG from...