The presence of a myeloid cell population showing strong reactivity with monoclonal antibody directed to difucosyl type 2 chain in epiphyseal bone marrow adjacent to joints affected with rheumatoid arthritis (RA) and its absence in the corresponding normal and non-RA bone marrow.

The presence of a specific type of cell, highly expressing difucosyl type 2 chain (dimeric Lex; FH-4 antigen), was found in the epiphyseal bone marrow cells of affected joints from patients with active severe rheumatoid arthritis (RA). The antigen was defined by monoclonal antibody FH-4 which was previously found to be directed to the oncofetal marker. The FH-4(+) cell population was identified as myeloid cell lineage with usual morphology specifically found in bone marrow of patients with severe RA, but is virtually absent in the same cell fraction of patients with osteoarthritis, infectious arthritis, and from normal adult subjects.