The invention provides ligands and fragments thereof to a receptor on the surface of activated CD4.sup.+ T-cells. An exemplary ligand is designated ACT-4-L-h-1. Preferred fragments include purified extracellular domains of ligands. The invention also provides humanized and human antibodies to the ligand. The invention further provides methods of using the ligand and the antibodies in treatment of diseases and conditions of the immune system. The invention also provides methods of monitoring activated CD4.sup.+ T-cells using the ligands or fragments thereof.