The term amyloidosis is used to describe a heterogeneous group of protein deposition diseases in which misfolding of proteins plays a prominent role.12 This process generates insoluble toxic fibrillar protein aggregates that are deposited in tissues in a characteristic p-pleated structure. These deposits are identified based on their apple-green birefringence under a polarizing light microscope after staining with Congo red dye, and by the presence of rigid nonbranching fibrils 7-10 nm in diameter on electron microscopy. Historically, the amyloidoses were classified as "primary" when no apparent etiology was evident, and "secondary" when resulting from chronic infectious or inflammatory states.3 In 1971, Glenner demonstrated the presence of immunoglobulin (Ig) light chains in primary amyloid fibrils.4 Since then, several other proteins have been characterized, and the chemical nature of the amyloidogenic protein forms the basis of the current classification (Table 90.1).5
Pathologically, all amyloid deposits share some common constituents, such as amyloid P component and glycosaminoglycans, but differ in the nature of the precursor protein. Clinically, the amy-loidoses are still classified as localized to a single organ, or systemic.6 The most common localized form is Alzheimer's disease, which affects more than 12 million people worldwide.7 In the western world, the two most common forms of systemic amyloidosis are AL (or Ig light chain) amyloidosis and reactive amyloido-sis due to chronic inflammatory diseases (e.g., rheumatoid arthritis). Hereditary amyloidosis is an ever expanding group of disorders that pose difficult diagnostic problems.8 In this chapter, we will largely focus on AL amyloidosis as it is associated with hematologic diseases.
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