Final answer:
A pathogen is an invader that triggers the body's immune response. Pathogens carry antigens that are recognized as foreign, and an infection or vaccination can activate the body’s immune cells, like B and T cells, to produce a targeted attack. The correct option is B.
Step-by-step explanation:
As soon as an invader, known as a pathogen, enters the body, the body begins to fight. When an infection occurs, an immediate inflammatory response occurs as soon as pathogens enter the body. Pathogens have proteins, often located on their cell surface, which are called antigens. These antigens are seen as foreign by the body and trigger an immune response.
The body prepares to fight off the pathogen by engaging B and T cells, which must be 'switched on' to produce an army of cells ready to combat that particular pathogen. Vaccination is a method to preemptively trigger this immune response by introducing a modified pathogen, ensuring that the body has 'memory' cells ready to more rapidly and effectively counteract an actual infection.