Which of the following is NOT a type of immune cell?

The correct answer is C, Pathogen, because a pathogen is not a type of immune cell; instead, it refers to an organism or agent that causes disease or illness in hosts, such as bacteria, viruses, fungi, or parasites. Immune cells, on the other hand, are specialized cells of the immune system that recognize and respond to pathogens in order to protect the body from infections.

Macrophages, T lymphocytes, and memory cells are all integral components of the immune response. Macrophages are large phagocytic cells that engulf and digest pathogens and dead or dying cells. T lymphocytes, or T cells, play a crucial role in the adaptive immune response, recognizing and attacking infected or cancerous cells. Memory cells are a subset of T cells or B cells that remain in the body after an infection, providing long-term immunity by "remembering" specific pathogens for quicker responses in future encounters. Hence, while the other options are types of immune cells, the inclusion of pathogen as an answer choice highlights the distinction between agents of disease and the cellular defenders of the immune system.