Rheumatoid factors are IgM and IgG antibodies. They react with altered endogenous IgG protein to form immune complexes in the joints. Affected dogs have episodes of anorexia, depression and fever with generalised or shifting lameness associated with swelling around the joints. It is not clear why the IgG protein changes to become recognised as foreign by the immune system. Many of the dogs affected are small and toy breeds, with a significant proportion being Shetland sheepdogs.
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