When we are exposed to antigens the body sends an immune response to prevent any infection or disease. Vaccines contain the same parts of antigens that can cause diseases, though they are weakened so that they don't actually cause disease, then they trigger the immune system to produce antibodies, which then help to fight against pathogens.
It introduces the body to a virus strand(s) to initiate internal resistance, which is created after a variety of metabolical processes. The idea is that the body would develop immunity to the virus without actually "getting" the virus.
Answer: Aristotle and Ptolemy had both assumed each planet is attached to a single sphere, but geometers and astronomers could employ a number of such spheres to generate the observed motions of the planets.
