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.

    Steel, concrete(混凝土) and glass are the usual materials in any skyscraper. But more high buildings are now using wood.

    In the last four years, Australia, Norway, England and Canada have all built wooden skyscrapers. The latest one is in Sweden, a new, 19-storey wooden hotel called "Sida vid Sida" (Side by Side).

    Why are wooden skyscrapers being chosen ahead of concrete ones? Let's find out.

    Is it strong enough?

    Wood has been used in many Chinese buildings for thousands of years, but is it strong enough for skyscrapers?

    "There's a lot of new materials made out of wood that are able to build big buildings," material expert Dr Michael Ramage told CNN when he was interviewed.

Architects (建筑师)have done thousands of experiment to make sure it is. They place layers of wood across one another coated with special glue to create very strong material.

Wood also lasts a long time and doesn't break down if it is well looked after. The examples include the rooftops of Beijing's Forbidden City and some 700-year-old houses in England.

Ramage also said on the internet that bamboo could be cut in special ways to make it even stronger than normal wood.

    Won't it catch fire?

    Wooden buildings sound dangerous when it comes to fire. But experts say they may be safer than concrete ones if there is a fire.

    "Wood doesn't burn in the way the public imagines," Ramage said. Very small pieces of wood catch fire easily, but very big pieces of wood are quite hard to set on fire, he said.

    Let me tell you something about sharks. Sharks are among the oldest animals in the world. There are over three hundred and fifty different kinds of sharks in the world. Most sharks are about two meters long and live in the seas.

    A shark can smell something in the water, for example, the blood of animals. That helps them find their food. Sharks mainly eat fish, other sharks and plants in the sea.

    Most sharks bite people by mistake. They think a person is a large sea animal. When the sun goes down or comes up, sharks are looking for food. So at these times people shouldn't go swimming in the sea.

    Today people kill sharks for food and their skin. The number of sharks in the world is dropping fast.

    How green are you? Do you know how to be green? We all need a healthy environment, but we produce waste every day and it is harmful to our environment. Though we are young, we can still do something to help. Here are some ideas for you.

    Reduce

    Reduce means "use less" .Don't waste things. This saves money and reduces pollution. Before we buy something new, think whether it is really necessary— or maybe the old one is still useful.

    Reuse

    Reuse means "use again" .When we buy things, make sure that they can last a long time. When something is broken, we should repair it instead of throwing it away and buying a new one. Don't use a paper cup or a paper bag. It's better to use a china(瓷) cup and a lunch box because you can use them again.

    Recycle

    Recycle means "change things into something else" .Though it takes energy to change something into something else, it's better than throwing things away or burning them.

    So please remember these three words: reduce, reuse and recycle.