Which of the following is NOT a component of the immune system?

Explanation: Memory cells are a type of immune cell that are formed during the body's initial encounter with a pathogen. They "remember" the specific antigen of the pathogen and allow the immune system to respond more rapidly and effectively if the same pathogen is encountered again in the future. Memory cells are responsible for providing long-term immunity.

Incorrect choices:

a. Engulfing pathogens and foreign invaders is the role of phagocytes, such as macrophages and neutrophils.

b. Initiating the inflammatory response is carried out by various immune cells, including mast cells and basophils, as part of the innate immune response.

c. Producing antibodies in response to infection is the primary role of B cells in the immune system.

Explanation: During cell-mediated immunity, T cells recognize antigens that are presented by antigen-presenting cells (APCs) such as macrophages. Macrophages engulf and process pathogens, and then they display fragments of the pathogens' antigens on their cell surface using major histocompatibility complex (MHC) molecules. T cells, specifically helper T cells, interact with these antigen-presenting macrophages and become activated to initiate immune responses.

Incorrect choices:
b. B cells present antigens to helper T cells during the humoral immune response, not during cell-mediated immunity.
c. Plasma cells are not involved in antigen presentation; they are terminally differentiated B cells that produce antibodies during the humoral immune response.
d. Memory cells are formed after the immune system encounters an antigen and are not directly involved in antigen presentation.