The immune system recognizes and kills pathogens and tumor cells to protect the host. The immune system is composed of two major subdivisions, the innate or non-specific immune system and the adaptive or specific immune system. Innate immunity that is constitutively present and is immediately mobilized upon infection acts as the first line of defense against invading organisms. It is non-specific and reacts equally well to a variety of organisms. While the adaptive immune system acts as a second line of defense, responding specifically and generating immunological memory. Both innate immunity and adaptive immunity have humoral response and cell-mediated responses. In the innate immune system, humoral responses involve cytokines, complement system, coagulation system, lysozymes and other secreted substances; cell-mediated responses involve a wide number of cell types, including phagocytes (neutrophiles, monocytes/macrophages, and dendritic cells), natural killer cells (NK cells), basophils, mast cells and eosinophils. In the adaptive or specific immune system, humoral responses (also called antibody responses) and cell-mediated responses are carried out by B cells and T cells, respectively.