<ref>Kumar, Vinay; Abbas, Abul K.; Fausto, Nelson; & Mitchell, Richard N. (2007). ''Robbins Basic Pathology'' (8th ed.). Saunders Elsevier. p.171 ISBN 978-1-4160-2973-1</ref>
<ref name=Robbins> Kumar, Vinay; Abbas, Abul K.; Fausto, Nelson; & Mitchell, Richard N. (2007). ''Robbins Basic Pathology'' (8th ed.). Saunders Elsevier. pp. 10-11 ISBN 978-1-4160-2973-1</ref>.