An allergen is a(n)

A. Naturally acquired active immunity occurs when the body is exposed to a pathogen, and the immune system produces its own antibodies in response. This type of immunity is not related to the transfer of antibodies from mother to fetus.
B. Artificially acquired active immunity involves vaccination, where the immune system is exposed to a weakened or inactivated pathogen to stimulate an immune response. This does not involve the transfer of antibodies from the mother.
C. Artificially acquired passive immunity involves the transfer of antibodies through medical interventions, such as antibody injections or serum treatments. While this involves passive immunity, it is not naturally occurring, nor is it related to placental transfer.
D. Naturally acquired passive immunity refers to the transfer of antibodies from mother to fetus through the placenta. These antibodies provide temporary protection to the newborn against infections, and this process is a natural example of passive immunity.

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.