Which white blood cell is known as the first line of defense after a microorganism has entered the body?

• A. Neutrophil
• B. Lymphocyte
• C. Monocyte
• D. Eosinophil

Answer: (A)

Neutrophils are the first responders of the innate immune system, arriving quickly at the site of infection to contain and begin destroying invading microorganisms. They’re highly efficient at phagocytosis, engulfing bacteria and then using oxidative burst and antimicrobial enzymes to kill them. This rapid mobilization is driven by chemotactic signals released during tissue injury and infection, so neutrophils act as the body’s immediate line of defense.

Lymphocytes belong to the adaptive immune system and require more time to become activated, providing targeted, specific responses. Monocytes also participate in phagocytosis but generally arrive after neutrophils and differentiate into macrophages in tissues. Eosinophils mainly target parasites and participate in allergic responses, not the immediate bacterial invasion.