ABSTRACT
Cellular immunity, as opposed to humoral immunity, encompasses a broad spectrum of immune phenomena mediated by several well-characterized cell populations. The latter comprises various subsets of lymphocytes: T, B, and natural killer (NK) cells, which are derived from a common lymphoid progenitor cell in the fetal liver and bone marrow. Monocytes, dendritic cells (DCs), and granulocytes represent hematopoietic cells with lineages distinct from that of lymphocytes, but which also participate in cellular immune reactions. They have the capability to mediate non-major histocompatibility complex (MHC)-restricted cytotoxicity and to release a variety of enzymes and cytokines. Lymphocytes interact with these other hematopoietic cells in the peripheral blood as well as in tissues, and the immune response represents a network of carefully balanced interactions responsible for maintaining homeostasis. A diagram of cells mediating innate and adaptive immunity is presented in Figure 5.1. Any perturbation of the immune network leads to a response-a series of events involving the immune effector cells-which is transient and which culminates in restoration of the baseline level of immune activity.
