The immune system protects the body against environmental pathogens and is divided into two subclasses: the innate (nonspecific) and the adaptive immune system. The adaptive or acquired immune system is characterized by specific responses toward each antigen and by enhanced responses after repeated antigen encounters (immune memory). Lymphocytes (T and B cells) are the main effector cells of the adaptive immune system. B cells are responsible for antibody production (humoral response), whereas helper T cells and T regulatory cells either directly destroy pathogens (cytotoxic T cells, i.e., CD8+T lymphocytes) or control the function of other cell types (helper T cells, i.e., CD4+T lymphocytes). Helper T cells can produce different cytokines, such as interferon-gamma (IFN-γ), tumor necrosis factor (TNF-α), and interleukins IL-4 and IL-5 (Meydani and Ha 2000).