Abstract: The present invention relates to anti-CD 103 antibodies, as well as use of these antibodies in diagnosis, prognosis, monitoring, and treatment of diseases. Also disclosed is an imaging agent comprising the anti-CD 103 antibody and a detectable label, wherein the antibody either does not block CD 103 binding to E-cadherin or at least partially blocks CD 103 binding to E-cadherin. The methods of treatment involve administering the anti CD 103 antibody which may be optionally coupled to a cytotoxic agent. Diseases to be treated include e.g. Hairy Cell leukemia, HCLv, intestinal and extraintestinal lymphomas, enteropathy-associated T-cell lymphoma (EATL), T-lymphoblastic leukemia/lymphoma (T-ALL), T-cell prolymphocytic leukemia (T-PLL), adult T cell leukemia/lymphoma (ATLL), mycosis fungoides (ME), anaplastic large cell lymphoma ALCL, cutaneous T-cell lymphoma (CTCL), Sezary Syndrome (SS), Alzheimer's disease, Parkinson's disease or multiple sclerosis.