Which organ lies in the right upper quadrant below the diaphragm, produces bile, and helps regulate immune responses?

The liver is the organ that lies in the right upper quadrant of the abdomen, just below the diaphragm. It plays a critical role in digestion and metabolism by producing bile, a substance essential for the emulsification and digestion of fats in the small intestine. In addition to its digestive functions, the liver is also key in regulating various immune responses. It contains immune cells, such as Kupffer cells, which help in the filtration of pathogens from the blood and contribute to the body's overall immune defense mechanisms.

The other options do not match all the criteria outlined in the question. The kidneys, located toward the lower back on either side of the spine, are primarily involved in filtering blood and producing urine. The spleen, which lies in the left upper quadrant, has immunological functions but does not produce bile. The gallbladder, while it is involved in the storage and concentration of bile produced by the liver, does not directly produce bile itself. Thus, the liver is the only organ listed that fulfills all the functions specified in the question.