To summarize, within the adaptive immune network, the CD4 T cell provides regulatory effects that play a central role in controlling the majority of immune functions and memory cells that enhance host protection. Many of the regulatory effects are mediated through the release of cytokines, a group of hormones that also promotes inflammation and replenishes hematopoietic elements. The CD8 T cell functions as a cytotoxic cell that is a crucial part of the host defense against intracellular pathogens such as viruses. The other major arm of the lymphoid network consists of B cells whose major function is the production of immunoglobulins that are crucial in host protection against encapsulated bacteria and other microbes. The role of the NK cell is less clear, but it appears to participate in host defenses against selected viruses and malignant cells. It also may be important in rejection reactions that are activated following tissue transplantation. The various lymphocyte types within the adaptive immune system are interdependent, as evidenced by the X-linked hyper-IgM syndrome in which a T-cell deficiency (CD40 ligand) manifests clinically as an antibody production problem.