Hairy Cell Leukemia
Hairy cell leukemia is a malignancy of uncertain classification, predominantly affecting elderly males. The clinical presentation is nonspecific, and includes malaise, fatigue, and frequent infectious episodes. The physical examination usually shows splenomegaly and sometimes generalized lymphadenopathy. The diagnosis is based on the finding of atypi-cal lymphocytes with numerous finger-like (or hairy) projections in the peripheral blood defining the disease (which derives its name from the morphological characteristics of the abnormal lymphocytes). The abnormal cells have mixed characteristics. On the one hand they express membrane immunoglobulins, often of several isotypes, and synthesize mono-typic heavy and light chains, suggesting a B-lymphocyte origin. However, after mitogenic stimulation, the abnormal lymphocytes may express the CD2 and CD3 membrane markers characteristic of T lymphocytes. On the other hand, they have monocyte/macrophage func-tions and markers including phagocytic properties, the ability to produce and release lysozyme and peroxidase, and the presence of intracellular tartrate-resistant acid phos-phatase. These findings have been interpreted as indicating the proliferation of B-cell pre-cursor cells sharing monocytic and T-cell markers and functions, malignant chimeric cells with multiple lineages, or malignant lymphocytes with aberrant gene expression. Without additional data it is impossible to decide which of these possibilities is more likely to re-flect accurately the nature of the malignant proliferation in this type of leukemia.
Interferon-α is therapeutically useful (sometimes inducing permanent remissions) in hairy cell leukemia. This seems to result from a direct antiproliferative effect that is at-tributed to the ability of interferon-α to promote redifferentiation of malignant cells, stop-ping their uncontrolled multiplication.