A and B are the iso-haemagglutmogens (isogens),
a and b are the corresponding iso-haemagglutimns (isonms).
The group is named after the iso-hasmagglutinogens (isogens).
The four groups indicate the only four possibilities, compatible with life, in \\hich the
iso-haemagglutinogens (isogens) and the iso-hsemagglutmins (isonins) can co-exist in the same
subject. In a 'defective' group an iso-h£emagglutimn (isonin) which can exist compatibly with life
is absent (e.g, O, a ; A, o).
Further division of A into Ax and A2 (and into At B and A2 B) increases the number of groups
There are difficulties of technique in determining A2 and A2. They are, therefore, ignored
by many workers m forensic medicine.
[By kind permission of Major (now Lieut.-Col.} S. D. S. Greval, I.M.S., and the Editor and Pub-
lishers of the Indian Journal of Medical Research.]
[To face page 98]