What is meant by pre-storage leukocyte-reduced red blood cells?

This form of red blood cells (RBCs) requires special preparation by removing leukocytes (white blood cells) by filtration shortly after donation. Studies show that blood transfusions with the leukocytes removed produce fewer side effects and reactions. This is done pre-storage because high numbers of leukocytes remaining in a unit of RBCs during the storage process can fragment, deteriorate, and release cytokines (chemicals that affect the inflammatory response). Leukocytes may be the cause of blood recipients developing reactions to current and subsequent blood transfusions.