Paraneoplastic neurologic syndromes (PNS) are a group of conditions that affect the nervous system (brain, spinal cord, nerves and/or muscles) in patients with cancer. The term "paraneoplastic" means that the neurological syndrome is not caused by the tumor itself, but by the immunological reactions that the tumor produces. It is believed that the body’s normal immunological system interprets the tumor as an invasion. When this occurs, the immunological system mounts an immune response, utilizing antibodies and lymphocytes to fight the tumor. The end result is that the patient’s own immune system can cause collateral damage to the nervous system, which can sometimes be severe. In many patients, the immune response can cause nervous system damage that far exceeds the damage done to the tumor. The effects of PNS can remit entirely, although there can also be permanent effects.