[Amyloidosis: definition and classification].

Amyloidoses are diseases characterized by deposits of altered proteins in the tissues. The amyloid deposit is always extracellular and presents a fibrillary conformation. 85% of the amyloid protein is constituted of a specific protein of each variety of amyloidosis, while the other 15% consists of other proteins and glycoproteins common to all types of amyloidosis. The same amyloid protein can be associated with various clinical forms and, inversely, different proteins can give the same clinical expression. It is therefore preferable to adopt a biochemical classification of amyloidosis. This classification is based on the identification of the various amyloid proteins by immunohistochemical analysis. About fifteen different amyloid proteins have already been identified. Hereditary amyloidoses are the most heterogeneous forms and amyloidoses of the nervous system are still under investigation. Immunohistochemical analysis of the majority of amyloidoses (represented by amyloidosis AA and AL) can now be performed routinely on tissue samples.