The indented area of the lymph node where efferent lymphatic vessels exit is called the

A. Lacteal is a lymphatic vessel located in the villi of the small intestine that absorbs fats, not the enlarged portion of the thoracic duct.
B. Hilum refers to the area on an organ where vessels, nerves, or ducts enter or leave, but it is not the enlarged portion of the thoracic duct.
C. The Right lymphatic duct is a separate duct that drains lymph from the right side of the body, not the enlarged portion of the thoracic duct.
D. Cisterna chyli is the enlarged, sac-like structure at the lower end of the thoracic duct, where lymph from the lower body and digestive organs (containing chyle) is collected before being transported into the thoracic duct.

A. Autoimmunity is a malfunction of the immune system where the body attacks its own tissues — it is not a type of protective immunity acquired through infection or exposure.
B. Species immunity refers to natural resistance to certain diseases that affect other species (e.g., humans not being susceptible to some animal viruses), not immunity acquired through infection.
C. Passive immunity involves receiving antibodies from another source (like from mother to baby or via antibody-containing injections) and does not require the immune system to produce its own response.
D. Active immunity is the correct answer. It develops when the body is exposed to a pathogen or its toxins and responds by producing its own antibodies and memory cells, providing long-term protection.