- Explain why allergies occur, and identify common allergens.
- Describe how autoimmune diseases affect the body.
- Define immunodeficiency, and list reasons for it.
- Explain how HIV is transmitted and how it causes AIDS.
- acquired immunodeficiency syndrome (AIDS)
- autoimmune disease
- human immunodeficiency virus (HIV)
Your immune system usually protects you from pathogens and keeps you well. However, like any other body system, the immune system itself can develop problems. Sometimes it responds to harmless foreign substances as though they were pathogens. Sometimes it attacks the body’s own cells. Certain diseases can also attack and damage the immune system and interfere with its ability to defend the body.
An allergy is a disease in which the immune system makes an inflammatory response to a harmless antigen. Any antigen that causes an allergy is called an allergen. Allergens may be inhaled or ingested, or they may come into contact with the skin. Two common causes of allergies are shown in Figure below. Inhaling ragweed pollen may cause coughing and sneezing. Skin contact with oils in poison ivy may cause an itchy rash.
Ragweed and poison ivy are common causes of allergies. Are you allergic to these plants?
The symptoms of allergies can range from mild to severe. Mild allergy symptoms are often treated with antihistamines. These are drugs that reduce or eliminate the effects of the histamines that cause allergy symptoms. The most severe allergic reaction is called anaphylaxis. This is a life-threatening response caused by a massive release of histamines. It requires emergency medical treatment. You can watch an animated video about how allergic reactions occur and how antihistamines can control them at this link: http://www.youtube.com/watch?v=y3bOgdvV-_M.
Autoimmune diseases occur when the immune system fails to recognize the body’s own molecules as “self,” or belonging to the person. Instead, it attacks body cells as though they were dangerous pathogens. Some relatively common autoimmune diseases are listed in Table below. These diseases cannot be cured, although they can be treated to relieve symptoms and prevent some of the long-term damage they cause.
Name of Disease
Tissues Attacked by Immune System
Results of Immune System Attack
tissues inside joints
joint damage and pain
Type 1 diabetes
insulin-producing cells of the pancreas
inability to produce insulin, high blood sugar
myelin sheaths of central nervous system neurons
muscle weakness, pain, fatigue
Systemic lupus erythematosus
joints, heart, other organs
joint and organ damage and pain
Why does the immune system attack body cells? In some cases, it’s because of exposure to pathogens that have antigens similar to the body’s own molecules. When this happens, the immune system not only attacks the pathogens. It also attacks body cells with the similar molecules.
Immunodeficiency occurs when the immune system is not working properly. As a result, it cannot fight off pathogens that a normal immune system would be able to resist. Rarely, the problem is caused by a defective gene. More often, it is acquired during a person’s lifetime. Immunodeficiency may occur for a variety of reasons:
- The immune system naturally becomes less effective as people get older. This is why older people are generally more susceptible to disease.
- The immune system may be damaged by other disorders, such as obesity or drug abuse.
- Certain medications can suppress the immune system. This is an intended effect of drugs given to people with transplanted organs. In many cases, however, it is an unwanted side effect of drugs used to treat other diseases.
- Some pathogens attack and destroy cells of the immune system. An example is the virus known as HIV. It is the most common cause of immunodeficiency in the world today. Compromised immune systems are discussed at http://www.youtube.com/watch?v=usRofaZEteY (2:36).
HIV and AIDS
Human immunodeficiency virus (HIV) is a virus that attacks the immune system. An example of HIV is shown in Figure below. Many people infected with HIV eventually develop acquired immune deficiency syndrome (AIDS). This may not occur until many years after the virus first enters the body.
HIV is a virus that attacks cells of the immune system.
HIV is transmitted, or spread, through direct contact of mucous membranes or body fluids such as blood, semen, or breast milk. As shown in Figure below, transmission of the virus can occur through sexual contact or the use of contaminated hypodermic needles. It can also be transmitted through an infected mother’s blood to her baby during late pregnancy or birth or through breast milk after birth. In the past, HIV was also transmitted through blood transfusions. Because donated blood is now screened for HIV, the virus is no longer transmitted this way.
HIV may be transmitted in all of the ways shown here. Based on how HIV is transmitted, what can people do to protect themselves from becoming infected? What choices can they make to prevent infection?
HIV and the Immune System
HIV infects and destroys helper T cells. As shown in Figure below, the virus injects its own DNA into a helper T cell and uses the T cell’s “machinery” to make copies of itself. In the process the T cell is destroyed, and the virus copies go on to infect other helper T cells. You can watch an animation showing how HIV infects T cells at this link: http://www.youtube.com/watch?v=9leO28ydyfU.
This diagram shows how HIV infects and destroys T cells.
HIV is able to evade the immune system and keep destroying T cells. This occurs in two ways:
- The virus frequently mutates and changes its surface antigens. This prevents antigen-specific lymphocytes from developing that could destroy cells infected with the virus.
- The virus uses the plasma membranes of host cells to hide its own antigens. This prevents the host’s immune system from detecting the antigens and destroying infected cells.
As time passes, the number of HIV copies keeps increasing, while the number of helper T cells keeps decreasing. The graph in Figure below shows how the number of T cells typically declines over a period of many years following the initial HIV infection. As the number of T cells decreases, so does the ability of the immune system to defend the body. As a result, an HIV-infected person develops frequent infections. Medicines can slow down the virus but not get rid of it, so there is no cure at present for HIV infections or AIDS. There also is no vaccine to immunize people against HIV infection, but scientists are working to develop one.
It typically takes several years after infection with HIV for the drop in T cells to cripple the immune system. What do you think explains the brief spike in T cells that occurs early in the HIV infection shown here?
AIDS is not a single disease but a set of diseases. It results from years of damage to the immune system by HIV. It occurs when helper T cells fall to a very low level and opportunistic diseases occur (see Figure above). Opportunistic diseases are infections and tumors that are rare except in people with immunodeficiency. The diseases take advantage of the opportunity presented by people whose immune systems can’t fight back. Opportunistic diseases are usually the direct cause of death of people with AIDS. You can watch a video showing when an HIV infection becomes AIDS at this link: http://www.youtube.com/watch?v=68I7JlVhuhY.
AIDS and HIV were first identified in 1981. Scientists think that the virus originally infected monkeys but then jumped to human populations, probably sometime during the early to mid-1900s. This most likely occurred in West Africa, but the virus soon spread around the world (see Figure below). Since then, HIV has killed more than 25 million people worldwide. The hardest hit countries are in Africa, where medicines to slow down the virus are least available. The worldwide economic toll of HIV and AIDS has also been enormous.
This map shows the number of people in different countries with HIV infections and AIDS in 2008. The rate of spread of the infection is higher Africa than in the U.S., yet the U.S. has a relatively large number of people with HIV infections and AIDS. Why might there be more survivors with HIV infections and AIDS in the U.S. than in Africa?
KQED: HIV Research: Beyond the Vaccine
Over the past 15 years, the number of people who die of AIDS each year in the United States has dropped by 70 percent. But AIDS remains a serious public health crisis among low-income African-Americans, particularly women. And in sub-Saharan Africa, the virus killed more than 1.6 million people in 2007. Innovative research approaches could lead to new treatments and possibly a cure for AIDS. HIV/AIDS has been described as a disease of poverty. Individuals with poor access to health care are less likely to see a doctor early on in their HIV infection, and thus they may be more likely to transmit the infection. HIV is now the leading cause of death for African American women between 24 and 35 years old.
For patients who have access to drugs, infection with the virus ceased to be a death sentence in 1995, when combinations of drugs called highly active antiretroviral therapy (HAART) were developed. For some patients, drugs can reduce the amount of virus to undetectable levels. But some amount of virus always hides in the body's immune cells and attacks again if the patient stops taking their medication. Researchers are working on developing a drug to wipe out this hidden virus, which could mean the end of AIDS. See http://www.kqed.org/quest/television/hiv-research-beyond-the-vaccine for further information.
- Allergies occur when the immune system makes an inflammatory response to a harmless antigen. An antigen that causes an allergy is called an allergen.
- Autoimmune diseases occur when the immune system fails to distinguish self from non-self. As a result, the immune system attacks the body’s own cells.
- In an immunodeficiency disease, the immune system does not work normally. As a consequence, it cannot defend the body.
- HIV is a virus that attacks cells of the immune system and eventually causes AIDS. It is the chief cause of immunodeficiency in the world today.
1. What is an allergen? Give two examples.
2. Define anaphylaxis. What causes the symptoms of anaphylaxis?
3. What is an autoimmune disease? Name an example.
4. List three possible reasons for acquired immunodeficiency.
5. Identify two ways that HIV can be transmitted.
6. Rheumatic fever is caused by a virus that has antigens similar to molecules in human heart tissues. When the immune system attacks the virus, it may also attack the heart. What type of immune system disease is rheumatic fever? Explain your answer.
7. Draw a graph to show the progression of an untreated HIV infection. Include a line that shows how the number of HIV copies changes through time. Include another line that shows how the number of helper T cells changes through time.
8. Sometimes people with an allergy get allergy shots. They are injected with tiny amounts of the allergen that triggers the allergic reaction. The shots are repeated at regular intervals, and the amount of allergen that is injected each time gradually increases. How do you think this might help an allergy? Do you think this approach just treats allergy symptoms or might it cure the allergy?
9. Explain why opportunistic diseases are a sign of immunodeficiency.
Points to Consider
Pathogens such as HIV are not the only cause of human disease. Many other things in our environment can also make us sick.
- Can you think of other environmental factors that negatively affect human health? What about pollutants in the environment? What are their possible health effects?
- Viruses cause some types of cancer, but cancer is more often caused by other environmental dangers. What environmental factors might increase the risk of cancer? Do you know what causes skin cancer, for example, or lung cancer?