Final answer:
Neutrophils are the type of leukocyte that is most active during a bacterial infection due to their role in the innate immune response and phagocytosis ability.
Step-by-step explanation:
The type of leukocyte that is very active during a bacterial infection is the neutrophil. These cells are a crucial part of the innate immune system and are typically the first responders to bacterial invasions. When an infection occurs, neutrophils are guided to the site of infection by various chemical signals where they perform phagocytosis to engulf and destroy the pathogens. Neutrophils are also known for releasing cytokines, which not only help in the destruction of pathogens but also signal other immune systems that an invasion has happened.
It is their ability to quickly respond and their abundance in the bloodstream that makes neutrophils the primary leukocytes to battle bacterial infections. In contrast, other leukocytes like lymphocytes are involved in the adaptive immune response, and eosinophils are more active against parasitic infections and allergic responses.