Vaccines and Our Immune System. Our immune system is an amazing and complex defense system that helps us fight off illnesses and infections. It works by recognizing foreign invaders such as bacteria and viruses, and then producing antibodies to destroy them. Vaccines are one of the most effective ways to help our immune systems fight off these invaders. Vaccines contain weakened or killed forms of a virus or bacteria, which stimulate our body’s natural defenses to create immunity against the disease without actually causing it. By receiving vaccines, we can protect ourselves from potentially deadly diseases and strengthen our immune systems for future threats.