Harmful microbes like bacterias, 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 specialized 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, their 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 sneaky 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 changed them into "serial Keller "cells to attack cancer cells. The study was small but the results were groundbreaking. The modified immune cells increased 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.

阅读理解

    Sixty years ago，human beings first reached the top of Mount Qomolangma， known as Mount Everest in the West. But the increase in climbers is turning the world's highest mountain into the “world's highest junkyard(垃圾场)”.

    Every year， more than 700 climbers spend nearly two months on Mount Qomolangma. They have left human waste and all kinds of rubbish， including oxygen bottles， broken tents， and plastic bags， according to Ang Tshering Sherpa， chief of the Nepal Mountaineering Association.

    “The waste has become a health danger to people in the area since they depend on water from rivers fed by the melting glaciers(融化的冰川)，” Sherpa said.

    More and more people have realized the danger that Mount Qomolangma is facing and they are now taking active actions. To deal with the problem， Nepal has supported a special team of explorers and volunteers to clean the mountain since 2008. So far they have brought down 15，000kg of rubbish. Nepal also started to ask each climber to bring down 8 kg of rubbish in 2014.

    On the Chinese side of the mountain， Tibet(西藏) will collect a cleaning fee of $100-200 (620-1240 yuan) per climber starting from this year. A program named “Mount Qomolangma Action at the Third Pole of the Earth” also started in 2004 to clear up the mountain.

    Hopefully， human beings will finally solve the problem， just as they found the way up the mountain successfully.

    Imagine a world with no more animals: no elephants, no tigers or lions. To some people, that thought is too terrible for words. So they are doing something about it. Tiffany Soechting is one of them.

    Being among wild animals—feeding them and caring for them—is all Tiffany Soechting has wanted to do with her life. And that is exactly what she does!

    Soechting is the human "mother" to the 500 animals that live on her family wildlife farm in San Antonio, Texas. On the farm, she cares for animals from around the world. Soechting says she loves them all. However, two reticulated giraffes(网纹长颈鹿) take up a special place in her heart. Their population in the wild has become smaller by 80% in the last 15 years. "I'm so glad to get help from five wildlife researchers that took giraffes up to a higher level. All our hard work helps protect giraffes living in the wild."

    Besides her animal caring, Soechting also holds monthly classes, where she teaches school children about the animals.

    Soechting also opens the farm to the public. Visitors not only learn about the animals, they can also get close to them. And the animals are not against that. For them, people are part of their natural environment.

The farm is part conservation(保护) area, part educational center and part animal park. As the animals walk peacefully around the grounds, it is easy for them to forget that San Antonio, a busy city, is just a few minutes away.

    Have you ever dreamed about a trip to the space? Seeds (种子) of some vegetables and crops are a lot luckier than you, as they have travelled to space and back.

    China is among the three countries in the world that carry out agricultural research (农业研究) in space. The others are Russia and the US.

    Since 1987, China has sent about 1, 000 species of seeds into space.

    These include green peppers, tomatoes, cucumbers (黄瓜), carrots, lettuce and wheat. Even on China's first manned spacecraft Shenzhou—V, there were some seeds from Taiwan.

    "Conditions in outer space such as radiation (辐射) and low gravity can cause big changes in the seeds," said Chinese researcher Jin Xing.

    But not all seeds are better after going to space. After returning to the Earth, the best seeds are selected. These seeds produce (生产) much more and also grow faster than normal (正常的) seeds