The C-type lectin DC-SIGN is absent in normal synovium but is highly expressed by CD68 positive macrophages in the synovium of rheumatoid arthritis patients. Accordingly, rheumatoid arthritis is diagnosed or treated by, respectively, assaying or blocking DC-SIGN. This can be accomplished by the use of agents, e.g., antibodies, which bind specifically to DCSIGN. Agents that bind to ICAM-3 are used to block inhibition of activation of macrophages by DC-SIGN-ICAM-3 interaction and cause inhibition of rheumatoid arthritis symptoms.