<P> The main response of the immune system to tumors is to destroy the abnormal cells using killer T cells, sometimes with the assistance of helper T cells . Tumor antigens are presented on MHC class I molecules in a similar way to viral antigens . This allows killer T cells to recognize the tumor cell as abnormal . NK cells also kill tumorous cells in a similar way, especially if the tumor cells have fewer MHC class I molecules on their surface than normal; this is a common phenomenon with tumors . Sometimes antibodies are generated against tumor cells allowing for their destruction by the complement system . </P> <P> Clearly, some tumors evade the immune system and go on to become cancers . Tumor cells often have a reduced number of MHC class I molecules on their surface, thus avoiding detection by killer T cells . Some tumor cells also release products that inhibit the immune response; for example by secreting the cytokine TGF - β, which suppresses the activity of macrophages and lymphocytes . In addition, immunological tolerance may develop against tumor antigens, so the immune system no longer attacks the tumor cells . </P> <P> Paradoxically, macrophages can promote tumor growth when tumor cells send out cytokines that attract macrophages, which then generate cytokines and growth factors such as tumor - necrosis factor alpha that nurture tumor development or promote stem - cell - like plasticity . In addition, a combination of hypoxia in the tumor and a cytokine produced by macrophages induces tumor cells to decrease production of a protein that blocks metastasis and thereby assists spread of cancer cells . </P> <P> The immune system is involved in many aspects of physiological regulation in the body . The immune system interacts intimately with other systems, such as the endocrine and the nervous systems . The immune system also plays a crucial role in embryogenesis (development of the embryo), as well as in tissue repair and regeneration . </P>

Which of these is not part of the immune system