Final answer:
Memory cells confer host protection against reinfection with the same pathogen by retaining information from the initial infection and mounting a quicker and more effective response upon subsequent exposures. Hence, option (A) is correct.
Step-by-step explanation:
The protection against reinfection with the same pathogen is mainly provided by memory cells. These cells are a critical component of the adaptive immune system. When the body is first infected with a pathogen, it triggers an immune response whereby B and T cells are activated to fight off the invaders.
After the infection is cleared, a subset of these cells develop into memory B and T cells. These memory cells retain information about the pathogen and ensure a faster and more effective immune response if the pathogen is encountered again. This immunity afforded by memory cells can be very long-lasting, protecting for years or even a lifetime.
Macrophages are phagocytic cells that can engulf pathogens and help promote inflammation, but they do not provide memory for future reinfections. Similarly, natural killer cells are involved in the innate immune response, especially against virally infected cells and tumors, but they do not have a memory function. Antigens are the substances that provoke an immune response, not cells that confer immunity.