The invention relates to members of the Fc receptor homolog (FcRH) subfamily, as well as fragments and variants thereof. Each FcRH is a Type I transmembrane receptor, preferably, comprises an extracellular region, a transmembrane region, and a cytoplasmic region. The cytoplasmic region preferably comprises one or more immunoreceptor tyrosine-based inhibitory or activation motifs ("ITIMs" or "ITAMs). The invention provides polypeptides, nucleic acids, vectors, expression systems, and antibodies and antibody fragments related to the FcRHs as well as uses thereof. Such uses include uses in the diagnosis and treatment of a malignancy of hematopoietic cell lineage or an inflammatory or autoimmune disease in a subject and in the modulation of a humoral immune response in a subject.