Monoclonal antibodies are identified that bind the IL-21 protein. These antibodies are used to identify regions of the IL-21 protein to where binding neutralizes IL-21 activity. Hybridomas and methods of producing anti-IL-21 monoclonal antibodies are described. The monoclonal antibodies are useful in treating IL-21-mediated diseases, which may include autoimmune and inflammatory diseases such as pancreatitis, type I diabetes (IDDM), Graves Disease, inflammatory bowel disease (IBD), Crohn's Disease, ulcerative colitis, irritable bowel syndrome, multiple sclerosis, rheumatoid arthritis, diverticulosis, systemic lupus erythematosus, psoriasis, ankylosing spondylitis, scleroderma, systemic sclerosis, psoriatic arthritis, osteoarthritis, atopic dermatitis, vitiligo, graft vs. host disease (GVHD), cutaneous T cell lymphoma (CTCL), Sjogren's syndrome, glomerulonephritis, IgA nephropathy, graft versous host disease, transplant rejection, atopic dermatitis, anti-phospholipid syndrome, and asthma, and other autoimmune diseases.

Methods useful for effecting prophylaxis or treatment of amyloidosis, including AA Amyloidosis and AL amyloidosis, by administering peptides comprising neoepitopes, such as AA fragments from a C-terminal region of AA, and antibodies specific for neoepitopes of aggregated amyloid proteins, for example, antibodies specific for the C-terminal region of AA fibrils. Antibodies for inhibition of formation and/or increasing clearance of amyloid deposits in a patient thus effecting prophylaxis or treating amyloid disease.