Immune Responses KEY CONCEPT The immune system has many responses to pathogens and foreign cells.

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1 SECTION 31.3 Immune Responses KEY CONCEPT The immune system has many responses to pathogens and foreign cells. Student text pages SC.912.L Many body systems work to produce nonspecific responses. When individual cells respond to pathogens, the response is called specific. Specific responses are different for each pathogen. But the body can also respond to pathogens in other, nonspecific* ways. Nonspecific responses are the same for every pathogen. These responses include inflammation and fever. Inflammation One kind of nonspecific response is inflammation. Inflammation is characterized by swelling, redness, pain, itching, and increased warmth. For example, if you scrape your knee, it will become inflamed. The body is trying to stop pathogens from entering through the newly broken skin. How does inflammation begin? First a pathogen enters the body. Then two kinds of white blood cells mast cells and basophils begin to respond. These cells release chemicals called histamines. Histamines stimulate the cells in blood vessel walls to spread out. Then fluid can move out of the blood vessels, into the nearby tissues. White blood cells squeeze out of the capillaries, moving toward the site of the infection. There, they can fight the pathogens. Once the pathogens are destroyed, the body repairs its tissues, stopping the inflammation. (The six types of white blood cells are shown in the chart below.) WHITE BLOOD CELLS NAME FUNCTION Basophil makes chemicals that cause inflammation in the bloodstream Mast cell makes chemicals that cause inflammation in other body tissues Neutrophil engulfs pathogens and foreign invaders; phagocyte There are six types of white blood cells. They help the body to trigger specific and nonspecific responses to pathogens. Macrophage Lymphocyte Eosinophil engulfs dead or damaged body cells and some bacteria; phagocyte destroys infected body cells or produces proteins that inactivate pathogens injects poisonous packets into parasites, such as protozoa * ACADEMIC VOCABULARY nonspecific general 416

2 Fever Like inflammation, fever is a general response to pathogens. It begins when mast cells or macrophages, another kind of white blood cell, release chemicals. These chemicals are carried by the blood to the hypothalamus, which controls the body s temperature. The hypothalamus signals the body to increase its internal temperature. Why does heat help the body to fight pathogens? A low fever, around 37.7 C (100 F), stimulates body cells to produce interferons. Interferons stop viruses from reproducing. Low fevers also cause white blood cells to mature* more quickly, so that there are more mature white blood cells available to destroy pathogens. With more white blood cells, the body can recover quickly from infection. WHITE BLOOD CELL MATURATION Low fevers help the body to fight infections, but high fevers are dangerous. A fever of more than 39 C (103 F) can prevent the hypothalamus from being able to control the body s temperature. Enzymes that control chemical reactions in the body stop working. A very high fever can cause seizures, brain damage, and even death. Underline Underline two ways two that ways inflammation that inflammation and fever and help fever the help body fight the pathogens. body fight pathogens. Cells of the immune system produce specific responses. The cells of the immune system can respond to pathogens in ways that are specific, or custom-made, for each pathogen. This response gives the body acquired immunity to the pathogen. For this to work, the body must be able to tell the difference between its own cells and pathogens. Antigens are protein markers on the surface of pathogens that white blood cells use to identify pathogens. When your body fights off an infection, it produces memory cells, which are specialized T and B cells that remember an antigen. If they meet this antigen a second time, they will quickly destroy it. As a result, the pathogen will not be able to reproduce and cause illness. There are two kinds of specific immune system responses that can cause the body to produce memory cells. The two responses are cellular immunity and humoral immunity. White blood cells mature faster when the body has a low fever (black line). * ACADEMIC VOCABULARY mature to become fully developed and able to function independently Interactive Reader 417

3 Cellular Immunity Cellular immunity is an immune respone that depends on T cells. Before T cells can destroy body cells that are infected by pathogens, they must be activated. This process is shown in the figure below. 1. A phagocyte swallows a pathogen. The pathogen s are removed and displayed on the phagocyte s cell membrane. The phagocyte is now called an antigen-presenting cell. 2. A T cell binds to this antigen-presenting cell, which releases proteins that activate the T cell. 3. The activated T cell divides into activated T cells and memory T cells. The activated T cells fights the infection. The memory T cells are kept in reserve for future infections. 4. The activated T cells bind to infected cells and cause them to burst. CELLULAR IMMUNITY 1 Phagocytes engulf pathogens and become antigenpresenting cells. pathogen 4 2 A T cell binds to the antigenpresenting cell, which activates the T cell. The activated T cells bind to infected body cells and cause them to burst. T cell 3 receptors The T cell divides into memory T cells and activated T cells. activated T cells memory T cells 418

4 Humoral Immunity Humoral immunity is an immune response that depends on B cells and antibodies. Before B cells can fight pathogens, they must be activated by T cells, as the figure below shows. 1. A pathogen binds to a B cell. The B cell engulfs the pathogen and puts part of the antigen on its surface. 2. A T cell binds to the antigen, releasing proteins to activate the B cell. 3. The activated B cell divides into activated and memory B cells. 4. Activated B cells produce up to 2000 pathogen-specific antibodies. Different antibodies fight pathogens by causing them to burst, inactivating them, or cause them to clump, as in the example below. 5. Phagocytes engulf and destroy the pathogen clumps. HUMORAL IMMUNITY 1 2 A pathogen binds to an inactivated B cell. The B cell puts part of the antigen on its own surface. B cell pathogen A T cell binds to the trapped antigen fragment and stimulates the B cell. T cell 5 Phagocytes eat the pathogen clumps. 4 The activated B cells produce antibodies that cause the pathogens to clump. antibodies 3 The B cell divides into memory B cells and activated B cells. memory B cells activated B cells In each box, highlight the step where memory cells are made. Interactive Reader 419

5 The immune system rejects foreign tissues. If you receive an organ transplant, your body must learn to treat the new cells as if they were its own. Otherwise, your immune system will attack the new cells and destroy them, a process called tissue rejection. If the new organ is from a close relative, the cells are more like your own. Your body would be less likely to attack them. To prevent tissue rejection, people who receive a new organ take drugs that make their immune systems less active. These drugs, however, also put people at risk from infections caused by viruses, bacteria, and fungi because, with a less active immune system, the body is less able to defend against pathogens. In some cases, the body stops recognizing its own cells and attacks them as foreign cells. Diseases caused by these attacks are called autoimmune diseases. As yet, scientists do not know what causes the body to attack its own cells. Why are people who receive new organs at risk for infections? 31.3 Vocabulary Check inflammation memory cells cellular immunity humoral immunity tissue rejection Mark It Up Fill in the blanks with the correct term from the list above. 1. occurs after T cells destroy infected body cells. 2. The protein markers that help the body tell the difference between its own cells and foreign cells are called. 3. are specialized T and B cells that provide acquired immunity by remembering. 4. Redness, itching, and warmth are examples of The Big Picture 5. Why is it important for the body to recognize its own cells? Go back and highlight each sentence that has a vocabulary word in bold. 6. Give an example of a nonspecific response to a pathogen. 420