What are Kupffer cells? Which organelle of Kupffer cells is most specialized?

Kupffer cells are specialised macrophages present in liver. They aid in the breakdown of old and worn out RBCs.

Lysosome is the most specialised organelle of Kupffer cells.
When old RBC enters these cells via endocytosis, then it combines with lysosomes of the kupffer cells to form phagosome. Inside the lysosome of Kupffer cells, RBC it is finally digested.