Plasma cells are key to the immune response because they secrete antibodies. given that antibodies are made of protein, which membrane-enclosed cell organelle would you expect the plasma cells to have in abundance?

The membrane-enclosed cell organelle that would be expected to have ribosome organelle in abundance.

RIBOSOME:

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I believe the answer is endoplasmic reticulum.

Plasma cell secrete antibodies which made of protein, so they will need many protein to do their role. Protein is macromolecule that made of multiple amino acids. Membrane-enclosed cell organelle that has an important role in protein synthesis is endoplasmic reticulum. Having many endoplasmic reticulum will help plasma cell to synthesis more antibodies.