After vaccination, our immune system is stimulated to produce antibodies even though we are not infected with any disease.
Vaccines trigger the immune system to produce its own antibodies, as if the body has been infected with a disease. This is called “ACTIVE IMMUNITY”.
If later on, the vaccinated person comes into contact with the disease itself, their immune system will recognize it and produce the antibodies needed to fight the disease.
As antibodies are passed from the mother to the newborn babies via the placenta, newborns are already protected against several diseases, such as measles, mumps and rubella. This is called “PASSIVE IMMUNITY”. It is called so because it only lasts for a few weeks or months. For example- In the case of measles, mumps and rubella, its antibodies may last up to one year (which is why the MMR jab is given to children just after their first birthday).