Harmful microbes like bacteris，viruses and other germs are pathogens．They may enter your bodies and make you ill．The immune system is the body's defence system that protects you from illness and disease caused by the pathogens．

The immune system is made up of non﹣specialized defences such as skin，tears and the acidic juice produced by the stomach，keeping you from the pathogens．It also has some highly specializde defences，which are different kinds of immune cells，grouped into phagocytes and lymphocytes．Phagocytes are a type of white blood cell．They are swimming around in your blood stream，always on the lookout for antigens﹣the pathogens that have entered your bodies．Whenever they see one，ther will eat it．However，some antigens are hard to find and kill．Then lymphocytes，which are also white blood cells and the main types are B cells，T helper cells and T killer cells，Will be the killers of the sncaky antigens．

B cells，T helper and T killer cells work together to find and kill these antigens．B cells produce substances，called antibodies，which stick to the antigen．When that happens，it will tell the T helper cells to come over and either：1．help the B cells kill the antigen，or 2．call the phagocytes or T killer cells to move in for the kill．Once a B cell or a T cell attacks an antigen，they create cells to"remember"it．Those"memory cells"hang out in your blood if they see the same antigen again，they quickly recognize，it so your immune system can act faster at killing it．

The immune system is more powerful than you probably imagine．Scientists from the University of Pennsylvania recently took immune cells from three patients with leukemia，then genetically cheanged them into "serial Keller" cells to attack cancer cells．The study was small but the results were groundbraking．The modified immune cells increascd at least 1，000 times in the body and killed all the cancer cells．Therefore，more "memory cells" were created．

The immune system can be developed．Eating lean protein food （beef，beans ）and food rich in vitamins A，C，and E（sweet potatoes，carrots ），taking a 30﹣minute walk every day and reducing your stress levels，etc．will help you achieve it．

37．What can the immune system do？　　

A．Keep you from antibodies．

B．Protect you against antigens．

C．Make your bodies werk and sick．

D．Help microbes enter your bodies

38．How are the sneaky antigens found and killed？　　

A．Phagocytes，B，T helper and T killer cells work together

B．Skin，tears，acidic juice and immune cells work together

C．B cells create "memory cells" to swim around in the blood

D．B cells produce antibodies to tell T helper and T killer cells

39．What does the underlined word "it" in the last paragraph refer to？　　

A．The immune system can be made more powerful

B．Less antigens can be killed by the immune system

C．White blood cells act less quickly in the immune system

D．The immune system can be changed into the "serial killer"

40．This article is mainly about　　．

A．the types of white blood cells

B．a research report about human healeh

C．the cause of illness and discase

D．an introduction to the immune system．