Which of the following is NOT a type of leukocyte?

Plasma cells are differentiated B lymphocytes that are responsible for producing antibodies in response to antigens. While they are derived from lymphocytes, they are not classified as a distinct type of leukocyte in the traditional sense; instead, they are a specialized form of lymphocyte with a specific function.

In contrast, lymphocytes themselves are indeed a type of leukocyte, serving critical roles in the immune response, including B cells and T cells. Phagocytes, such as macrophages and neutrophils, are another category of leukocytes that ingest and digest pathogens and debris. Auxiliary cells, typically associated with helper T cells, play a supportive role in the immune response but do not refer to a well-defined category within leukocytes in the way lymphocytes and phagocytes do.

The distinction here lies in the fact that while plasma cells arise from lymphocytes, they represent a specialized function rather than a separate category like the other options. Therefore, identifying plasma cells as not a distinct type of leukocyte aligns with an understanding of the functional diversity within the immune system.