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Medical Definition of Serum amyloid
1. In secondary amyloidosis the fibrils deposited in tissues are unrelated to immunoglobulin light chains (in contrast to the situation in primary amyloidosis) and are made of amyloid A protein (AA protein). This is derived from serum amyloid A (SAA) that is the apolipoprotein of a high density lipoprotein and an acute phase protein. Partial proteolysis converts SAA into the pleated sheet configuration of the amyloid fibrils. Amyloid P protein is also found as a minor component of the fibrils (in both primary and secondary amyloidosis) and is derived from serum amyloid P that has similarity to C-reactive protein. The physiological role remains obscure. This entry appears with permission from the Dictionary of Cell and Molecular Biology (11 Mar 2008)