What is the molecule that stimulates an immune response called?

The principle of Vaccination and Immunization is based on which property of the immune system?

In higher vertebrates, the immune system can distinguish between self and non-self cells. Due to genetic or other unknown reasons, the immune system sometimes loses this property and starts attacking self-cells, resulting in:

Where are $B$-cells and $T$-cells formed? How do they differ from each other?

Grafted kidney may be rejected in a patient due to

What constitutes the immune system and what role does it play in the human body?