Antibodies respond to foreign proteins (bacteria, parasites, etc) and foreign matter injected into the body as a vaccine. In case you missed them, you can find articles I've written on the different antibodies — IgG, IgA, IgM, IgE and IgD — at their respective links.

Antibodies are part of the adaptive immune system. Other than antibodies transferred from the mother to a baby in utero, the immune system begins to adapt as soon as the baby is exposed to particles in the environment.

The other arm of the immune system is known as the innate immune system.  Innate immunity is the body's first line of defense designed to prevent infection, eliminate invading pathogens, and stimulate the acquired immune response.