What is the primary role of memory cells in the immune system?

Explanation: The immune response is a complex system of defense mechanisms that aim to protect the body from pathogens, such as bacteria, viruses, and other foreign substances. It involves the activation of various immune cells, production of antibodies, and other immune components to recognize and eliminate the invading pathogens.

Incorrect choices:

a. Directly attacking and killing infected cells is the role of cytotoxic T cells during cell-mediated immunity, but it is not the primary purpose of the overall immune response.

b. Antigens are foreign substances that trigger the immune response. They are not produced by the immune response itself but rather recognized by immune cells and antibodies.

c. The immune response is not involved in regulating the body's metabolism. Metabolism regulation is primarily controlled by other systems in the body, such as the endocrine system.

Explanation: Active immunity is acquired when the body's immune system is exposed to a pathogen through vaccination or natural infection and produces an immune response to provide protection against future infections.

Incorrect choices:

b. Passive immunity is acquired when preformed antibodies are transferred to an individual, providing immediate but temporary protection against a specific pathogen.

c. Natural immunity refers to immunity that is acquired through natural exposure to a pathogen, either through infection or vaccination.

d. Herd immunity is the indirect protection that occurs when a large percentage of a population becomes immune to a disease, either through vaccination or previous infection, reducing the likelihood of transmission to susceptible individuals.