To understand the difference between immunity after infection and immunity after vaccination, we need to understand the way the body reacts to a foreign body, in this case the SARS-COV-2 Virus. When a Virus enters the body, the Antigen Presenting Cells (APC) recognize it, engulf it and break it down to its components. These components are then expressed on the surface of the APC to activate a part of the immune system called T- Lymphocytes.