How do vaccines work?

Pathogens can cause diseases. We can prevent a pathogen from causing a disease in an individual by giving them a vaccine which can make them immune to the pathogen. We do this by taking a dead/weakened form of the pathogen, or only part of the pathogen, and injecting it into the person. This causes lymphocytes (specifically B lymphocytes, which are types of white blood cells) to produce antibodies that are complementary to the antigen injected. These antibodies cause the pathogen to clump together, allowing other white blood cells called phagocytes to engulf and destroy them (phagocytosis). This is called the primary response.Some B lymphocytes remain in the bloodstream as memory cells. These memory cells are very good at producing the antibodies complementary to the antigen previously injected. This means when the pathogen is encountered again, appropriate antibodies are produced much faster and in higher amounts, allowing the pathogen to be dealt with so quickly that most people don't even get symptoms of the disease. This is the secondary response.