Michelle just finished her last day of fourth grade when Justin sneezed on her right before she got off. She didn't think anything of it but her body already knew what was about to come.
Job is done!
The fist line of defense would be Michelle's skin, nose hair, and mucus. Her skin is the protective out layer against foreign invaders, her nose hair helps filter the air and traps foreign particles, and the mucus in her lungs and other inner surfaces helps trap and expel foreign objects.
When Anthony the Antibody detect the villain antigen it tells the Tony the Phagocytes; Tony begins to eat the unwanted substances.
Tony the phagocyte breaks down the antigen and presents fragments of the antigen protein on the surface of a cell membrane.
Timmy t-cell has surface receptor sites that match to the villain antigen and connect to it and immune response is switched on by Timmy.
Timmy the T-cell now produces a chemical that brings Bobby B-cell to the situation.