B and T cells are:

A. Chyle drainage is not a function of the spleen. Chyle, a milky fluid rich in fats absorbed from the intestines, is transported by the lymphatic system — specifically through structures like lacteals and the thoracic duct — not the spleen.
B. Blood filtration is a key function of the spleen, as it removes pathogens and debris from the blood.
C. Destruction of old red blood cells occurs in the spleen, which acts as a graveyard for aging or damaged red blood cells.
D. Blood storage is also a spleen function; it can store blood and release it in case of hemorrhage or increased demand.

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.