The enlarged portion of the thoracic duct is called the:

A. Lymph nodes are secondary lymphoid organs where B and T cells can be activated, but they are not the cells themselves.
B. Tonsils are lymphoid tissues that help protect against pathogens, but they are not the cells themselves.
C. Lymphocytes are the correct answer. B cells and T cells are types of lymphocytes, which are white blood cells involved in the adaptive immune response.
D. Antibodies are proteins produced by B cells (specifically plasma cells) in response to pathogens, but they are not cells themselves.

A. Neutrophils are a type of white blood cell that primarily responds to bacterial infections and perform phagocytosis, not specifically attacking tumor or virus-infected cells.
B. Regulatory T cells help regulate the immune response and prevent excessive immune activity, but they do not directly attack tumor or virus-infected cells.
C. Natural killer (NK) cells are a type of lymphocyte that plays a key role in the body's defense against tumor cells and virus-infected cells by directly attacking and destroying them.
D. Memory T cells are responsible for "remembering" a previous pathogen, enabling a quicker response upon future exposure, but they do not directly attack tumor or virus-infected cells.