What happens to immune cells when a pathogen is detected in the body?

When a pathogen is detected in the body, immune cells respond by multiplying rapidly. This increase in cell number is crucial for mounting an effective immune response against the invading pathogen. When immune cells, such as T cells and B cells, recognize specific antigens from the pathogen, they become activated. Activation leads to clonal expansion, where these cells replicate to produce a large number of identical cells that can specifically target and eliminate the pathogen.

The rapid multiplication ensures that there are enough immune cells available to combat the infection, providing a more robust defense. This process is part of both the innate and adaptive immune responses, with the latter being particularly reliant on the proliferation of B and T lymphocytes tailored to the specific antigens presented by the pathogen.

The other options do not accurately reflect the immediate response of immune cells to pathogen detection. For example, becoming dormant is counterproductive during an infection, while changing function or undergoing apoptosis does not directly describe the primary initial response of immune cells to effectively address the threat.