The JMH blood group was first identified by autoantibodies usually found in elderly men whose red cells exhibited a positive direct antiglobulin test. When found in women they too were elderly and were often cat owners; hence the earlier nicknames of this collection of specificities was "The Boys" and "Cat". The name was eventually assigned based upon the initials of one of the most thoroughly investigated propositi. Only one family to date has been shown to have an inherited JMH negative phenotype while the majority of the JMH negatives are due to an acquired suppression of the antigen. As with many of the early reports, anti-JMH was believed to be a single specificity. However, the Meged serum (and later others) showed that other polymorphic determinants were involved in the JMH phenotype.

The JMH antigen is a 76 kD protein as identified by both human and monoclonal antibodies. In addition to being found on red cells it is also present on leukocytes and it has been given the cluster determination CDw108. This protein is known to be acquired during lymphocyte activation and may play a role in cell adhesion. Since it is a GPI-anchored protein, the JMH antigens are absent from PNH red blood cells as are some of the other blood group bearing molecules, eg. decay accelerating factor (Cromer).