The present invention relates to novel LRF-1 and LRF-2 proteins which are related to the CRISP family and a protein called "Neutrophil Inhibitory Factor (NIF)" isolated from the canine hookworm (Ancylostoma caninum) that potently inhibits CD11/CD18-dependent neutrophil function. In particular, isolated nucleic acid molecules are provided encoding the human LRF-1 and LRF-2 proteins. LRF-1 and LRF-2 polypeptides are also provided, as are vectors, host cells and recombinant methods for producing the same. The invention further relates to screening methods for identifying agonists and antagonists of LRF-1 or LRF-2 activity. Also provided are diagnostic methods for detecting immune system or other LRF-1- or LRF-2-related disorders and therapeutic methods for treating such disorders.