Any foreign substance that enters the body an induces an immune response is called a(n):

A. Jaundice is caused by a buildup of bilirubin in the blood, typically due to liver or bile duct issues, not a blockage of lymph flow.
B. Edema occurs when there is an accumulation of fluid in tissues, which can result from a blockage in the lymphatic system (called lymphedema). This disrupts the normal drainage of lymph, causing swelling.
C. Varicose veins are swollen, twisted veins, typically caused by poor blood circulation, not lymphatic blockage.
D. Anemia is a condition where there is a deficiency of red blood cells or hemoglobin, and it is not related to a blockage of lymph flow.

A. Killer T cells (also known as cytotoxic T cells) recognize and bind to antigens presented by infected cells. They initiate a rapid immune response to destroy the infected or cancerous cells directly.
B. Regulatory T cells help modulate the immune response by preventing overreaction or autoimmunity, but they do not directly initiate a rapid response to antigens.
C. Memory T cells remember previously encountered antigens and respond quickly upon re-exposure to the same pathogen, but they do not initiate the initial response—they are involved in secondary immune responses.
D. Thrombocytes (also known as platelets) are involved in blood clotting, not in the immune response to antigens.