Amyloid diseases are defined by the biochemical nature of the protein in the fibril deposits and are classified according to whether they are systemic or localized, acquired or inherited, and their clinical patterns. The accepted nomenclature is AX where A indicates amyloidosis and X is the protein in the fibril (Table 137-1, p. 720, in HPIM-19).

• AL (immunoglobulin light chains): primary amyloidosis; most common form of systemic amyloidosis; arises from a clonal B-cell disorder, usually multiple myeloma.
• AA (serum amyloid A): secondary amyloidosis; can occur in association with almost any chronic inflammatory state (e.g., RA, SLE, periodic fever syndromes such as familial Mediterranean fever [FMF], Crohn's disease) or chronic infections.
• AF (familial amyloidoses): number of different types that are dominantly transmitted in association with a mutation that enhances protein misfolding and fibril formation; most commonly due to transthyretin.
• Aβ₂M: composed of β₂ microglobulin; occurs in end-stage renal disease of long duration.
• Localized or organ-limited amyloidoses: most common form is Aβ found in Alzheimer's disease derived from abnormal proteolytic processing of the amyloid precursor protein.