This document discusses the preparation and importance of check cells, which are red blood cells that have IgG antibodies attached to their surface. It describes how check cells are made by pooling O+ red blood cells from multiple donors and incubating them with IgG antibodies to allow the antibodies to attach. Check cells are used as controls in tests like the Coombs test to validate the test results. They help determine if a negative Coombs test is a true negative or a false result. Check cells can be stored for 7 days at 2-6°C or 24 hours at 25°C and are important for validating reagents, tests, and detecting antibodies in other tests.