Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is the most common form of leukemia in the United States. This prevalence is due to two factors. First, the incidence of CLL rises with advancing age with the median age in the sixties. As noted previously, the population over the age of 60 continues to grow. Second, patients with CLL can experience a long survival, with median survival approaching 6 years. Thus, the prevalence of the disease is quite high.

CLL is generally a neoplasm of activated B lymphocytes. The leukemic cell morphologically resembles mature small lymphocytes normally seen in the peripheral blood. A sustained blood lymphocyte count that is greater than 10 × 10³/mL has been required to diagnose CLL. Additionally, 30% of nucleated cells in the bone marrow should be lymphocytes, and the peripheral blood lymphocytes should be positive for B-cell markers. In some clinical situations, CLL can be diagnosed with fewer total lymphocytes if surface markers confirms a monoclonal population of typical lymphocytes.

The exact cell of origin in CLL is unknown. Although 95% of patients have a B-cell phenotype, T-cell CLL can rarely occur. The typical B-cell CLL does stain positive with antibody to CD5, which is normally a T-cell antigen, and this has been taken to imply that the malignant cell is arrested at an intermediate stage of B-cell differentiation. Other diseases, such as hairy cell leukemia, to be considered later, are thought to result from the malignant transformation of a lymphocyte at a different stage of development.

The etiology of CLL is also unknown. Retroviruses and ionizing radiation, which lead to other forms of leukemia, apparently do not increase the risk for CLL. Interestingly, the incidence of CLL is far less in Japan than in Western populations. In addition, there is an increased risk of a variety of autoimmune disorders in relatives of patients with CLL, implying a genetic predisposition to the development of lymphocytic disorders.