1. [n]  a bachelor's degree in arts and sciences
2. [n]  the blood group whose red cells carry both the A and B antigens
3. [n]  the eleventh month of the civil year; the fifth month of the ecclesiastical year in the Jewish calendar (in July and August)

AB is a 2 letter word that starts with A.

1. \Ab-\ ([a^]b). [Latin prep., etymologically the same as E.
   of, off. See {Of}.]
   A prefix in many words of Latin origin. It signifies from,
   away, separating, or departure, as in abduct, abstract,
   abscond. See {A-}(6).
   

2. \Ab\ ([a^]b), n. [Of Syriac origin.]
   The fifth month of the Jewish year according to the
   ecclesiastical reckoning, the eleventh by the civil
   computation, coinciding nearly with August. --W. Smith.
   

1. The portion of the immunoglobulin or antibody molecule which contains the binding site for antigens. The exact sequence of amino acids in the area varies widely from molecule to molecule to accommodate a wide variety of antigens which the body may encounter. There are two such regions on each molecule (individually called Fab fragments or F-ab fragments). The molecule is shaped like the letter Y and the F(ab)2 fragments are located on the upper halves of the two fork parts. (The rest of the molecule, the stem and the lower parts of the forks, are the Fc fragment).
2. A fragment of an antibody protein which includes the antigen-binding portions but not the Fc section. They can be produced by treating whole antibodies with proteases that will specifically cleave off the Fc section.