AB, Ab, or ab may refer to:

• A. B. is a pen name of Aubrey Beardsley
• A. B. is a pen name of Benjamin Franklin
• A. B. is a pen name of Jonathan Swift

is a 13-episode Japanese anime television series produced by P.A.Works and Aniplex and directed by Seiji Kishi. The story was originally conceived by Jun Maeda, who also wrote the screenplay and composed the music with the group Anant-Garde Eyes, with original character design by Na-Ga; both Maeda and Na-Ga are from the visual novel brand Key, who produced such titles as Kanon, Air, and Clannad. The anime aired in Japan between April 3 and June 26, 2010. An original video animation (OVA) episode was released in December 2010, and a second OVA was released in June 2015. The story takes place in the afterlife and focuses on Otonashi, a boy who lost his memories of his life after dying. He is enrolled into the afterlife school and meets a girl named Yuri who invites him to join the Afterlife Battlefront, an organization she leads which fights against the student council president Angel, a girl with supernatural powers.

A blood type (also called a blood group) is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group system. Some of these antigens are also present on the surface of other types of cells of various tissues. Several of these red blood cell surface antigens can stem from one allele (or an alternative version of a gene) and collectively form a blood group system. Blood types are inherited and represent contributions from both parents. A total of 35 human blood group systems are now recognized by the International Society of Blood Transfusion (ISBT). The two most important ones are ABO and the RhD antigen; they determine someone's blood type (A, B, AB and O, with +, - or Null denoting RhD status).