Immunity is the body's ability to recognize, destroy, and eliminate foreign antigens like bacteria and viruses. Antigens induce an immune response that leads to the formation of antibodies that specifically react with that antigen. Antibodies are immunoglobulins, including IgG, IgM, IgD, IgA, and IgE, that react with the antigens that stimulate their production. Vaccines are generally safe and effective, but all immune responses carry some risk of adverse reactions, like minor local reactions, irritability, fever or gastrointestinal upset. Herd immunity occurs when vaccinating a portion of a population provides indirect protection to unvaccinated individuals by making it difficult for diseases to spread from person to person.