The immune reaction is your body's response to pathogens such as Bacteria or Viruses.

When a pathogen enters your body, specialized defense cells recognize it as "foreign".

Defense substances (antibodies) and other protective mechanisms are then activated to combat the invader.

Some defense cells engulf and eat the pathogens, while others destroy infected body cells.

After successful defense, the immune system "remembers" the pathogen to react more quickly on the next encounter.

There is an innate defense which acts immediately and an adaptive defense which is more targeted but takes a bit longer.

This immune reaction protects you from becoming ill or helps you recover.