Introduction
The immune system encompasses a complex network of cells and signaling molecules crucial to the defense against microorganisms and specific forms of cancer. It includes several different subsystems, and many cell types, such as lymphocytes, macrophages, granulocytes, and dendritic cells, each with their own functional subsets. The normal immune response involves a variety of processes, including cell activation and proliferation in response to a variety of ligands recognized by specific receptors, as well as mechanisms of termination of the response when it is no longer necessary. This complexity results in a large variety of diseases if the function of one or more of these cell types, and one or more of the processes essential to immune homeostasis, is affected. As a result, immunodeficiencies with a wide range of clinical manifestations, from complete inability to mount a proper response to infections, to uncontrolled ongoing immune responses, develop. This chapter addresses lymphoproliferative disorders that are a consequence of immunodeficiency.
Immunodeficiency is associated with an increased risk of benign and malignant lymphoproliferative diseases (LPDs). These may result from aberrant reactions to normal stimuli, the complete inability to terminate an immune response, or accumulation of genetic defects in the immune cells. Biopsies from patients with immunodeficiencies pose a challenge to the pathologist, due to the aberrant make-up of the immune response in this context, the different nature of the lymphoproliferations, and the altered morphology of lymphomas arising in this setting, when compared to immunocompetent hosts.