A vaccine is a biological preparation designed to stimulate an immune response against a specific pathogen. These preparations introduce antigens—components of the pathogen, which can be deactivated viruses, mRNA sequences, or specific protein fragments—into the body. The presence of these antigens allows the immune system to recognize the threat and mount a protective response without causing the actual disease. This initial exposure triggers the production of antibodies and the establishment of memory B and T cells. Should the organism encounter the live, active pathogen later, the immune system can rapidly deploy these stored defenses, mitigating the likelihood of severe illness.