Rituxan is a mouse antibody that has been changed to make it similar to a human antibody. Antibodies are proteins that can protect the body from foreign invaders, such as bacteria and viruses, by binding to substances called antigens. Rituxan works by binding to a protein, called the CD20 protein (which is found on the surface of normal and cancerous B-lymphocytes). This binding may help to destroy the B-lymphocytes.