18.6: Disorders of the Immune System

Allergy Eyes

Eyes that are red, watery, and itchy are typical of an allergic reaction known as allergic rhinitis. Commonly called hay fever, allergic rhinitis is an immune system reaction typically to the pollen of certain plants. 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. That’s the basis of allergies such as hay fever.

Figure \(\PageIndex{1}\): Oedema of the conjunctiva due to hay fever allergy. Oedema, Championswimmer CC0

Allergies

An allergy is a disorder in which the immune system makes an inflammatory response to a harmless antigen. It occurs when the immune system is hypersensitive to an environmental antigen that elicits little or no response in most people. Allergies are strongly familial: allergic parents are more likely to have allergic children, and those children’s allergies are likely to be more severe. This provides evidence of a heritable tendency to develop allergies. Allergies are more common in children than in adults because many children outgrow their allergies by adulthood.

Allergens

Figure \(\PageIndex{2}\): Common causes of allergies include pollen and poison ivy. Pollen, Hans via Pixabay and Poison Ivy, Sam Fraser-Smith CC-BY 2.0

Any antigen that causes an allergy is called an allergen. Common allergens are plant pollens, dust mites, mold, specific foods (such as peanuts or shellfish), insect stings, and certain common medications (such as aspirin and penicillin). Allergens may be inhaled or ingested, or they may come into contact with the skin or eyes. Symptoms vary depending on the type of exposure and the severity of the immune system response. Two common causes of allergies are ragweed and poison ivy. Inhaling ragweed pollen may cause symptoms of allergic rhinitis, such as sneezing and red, itchy eyes. Skin contact with oils in poison ivy may cause an itchy rash. This type of allergy is called contact dermatitis.

Prevalence of Allergies

There has been a significant increase in the prevalence of allergies over the past several decades, especially in the rich nations of the world, where allergies are now very common disorders. In the developed countries, about 20 percent of people have or have had hay fever, another 20 percent have had contact dermatitis, and about 6 percent have food allergies. In the poorer nations of the world, on the other hand, allergies of all types are much less common.

One explanation for the rise in allergies in the developed world is called the hygiene hypothesis. According to this hypothesis, people in developed countries live in relatively sterile environments because of hygienic practices and sanitation systems. As a result, people in these countries are exposed to fewer pathogens than their immune system evolved to cope with. To compensate, their immune system “keeps busy” by attacking harmless antigens in allergic responses.

How Allergies Occur

Figure \(\PageIndex{3}\): This diagram shows how the adaptive immune system is activated by an otherwise harmless antigen on ragweed pollen, responding to the allergen as though it were a pathogen. Mast Cells, NIH CC0

The diagram in Figure \(\PageIndex{3}\) shows how an allergic reaction occurs. At the first exposure to an allergen, B cells are activated to form plasma cells that produce large amounts of antibodies to the allergen. These antibodies bind to mast cells. Subsequently, whenever the person encounters the allergen again, the mast cells are already primed and ready to respond. The primed mast cells immediately release cytokines and histamines, which, in turn, trigger inflammation and leukocyte recruitment, among other responses. These responses are responsible for the signs and symptoms of allergies.

Treating Allergies

The symptoms of allergies can range from mild to life-threatening. Mild allergy symptoms are often treated with antihistamines. These are drugs that reduce or eliminate the effects of the histamines that produce allergy symptoms.

Figure \(\PageIndex{4}\): Anaphylaxis is a rapid, systemic reaction to allergens that may lead to life-threatening symptoms. Signs and symptoms of anaphylaxis, Mikael Häggström CC0

Treating Anaphylaxis

The most severe allergic reaction is anaphylaxis, a systemic reaction. This is a life-threatening response caused by a massive release of histamines. Many of the signs and symptoms of anaphylaxis are shown in Figure \(\PageIndex{4}\). Some of them include a drop in blood pressure, changes in heart rate, shortness of breath, and swelling of the tongue and throat, which may threaten the patient with suffocation unless emergency treatment is given. People who have had anaphylactic reactions may carry an epinephrine autoinjector (widely known by its brand name, EpiPen®) so they can self-inject epinephrine if they start to experience an anaphylactic reaction. Epinephrine helps control the immune response until medical care can be provided. Epinephrine constricts blood vessels to increase blood pressure, relaxes smooth muscles in the lungs to reduce wheezing and improve breathing, modulates heart rate, and reduces swelling that may otherwise block the airways.

Immunotherapy for Allergies

Another way to treat allergies is called immunotherapy, also known as “allergy shots.” This approach may actually cure specific allergies, at least for several years if not lifelong. It may be particularly beneficial for allergens such as pollen that are difficult or impossible to avoid. First, however, patients must be tested to identify the specific allergens causing their symptoms. As shown in Figure \(\PageIndex{5}\), this may involve scratching tiny amounts of common allergens into the skin and then observing whether there is a localized reaction to any of them. Each allergen is applied to a different numbered site on the skin, so if there is a reaction, such as redness or swelling, the responsible allergen can be identified. Then, through periodic injections (usually weekly or monthly), patients are gradually exposed to increasing amounts of the allergens. Over time, generally from months to years, the immune system becomes desensitized to the allergens. This method of treating allergies is often effective for pollen or insect-sting allergies, but its usefulness for food allergies is unclear.

Figure \(\PageIndex{5}\): Skin testing for common allergens is one way to identify the cause(s) of a patient’s allergic symptoms. Skin prick test for allergies, NIH CC0

Autoimmune Diseases

Autoimmune diseases occur when the immune system fails to recognize the body’s own molecules as self. As a result, instead of ignoring the body’s healthy cells, it attacks them, causing tissue damage and altered organ growth and function. Most often, B cells are at fault in autoimmune responses. They are generally the cells that lose tolerance for self. Why does this occur? Some autoimmune diseases are thought to be caused by exposure to pathogens that have antigens similar to the body’s own molecules. After this exposure, the immune system responds to body cells as if they were pathogens.

Certain individuals are genetically susceptible to developing autoimmune diseases. These individuals are also more likely to develop more than one such disease. Gender is also a risk factor for autoimmunity. Females are much more likely than males to develop autoimmune diseases, probably in part because of gender differences in sex hormones.

At a population level, autoimmune diseases are less common than infectious diseases. The hygiene hypothesis has been proposed to explain the inverse relationship between infectious and autoimmune diseases, as well as the prevalence of allergies. According to the hypothesis, without infectious diseases to “keep it busy,” the immune system may attack the body’s own cells instead.

Common Autoimmune Diseases

An estimated 15 million or more people worldwide have one or more autoimmune diseases. Two of the most common autoimmune diseases are type I diabetes and multiple sclerosis. Both are localized diseases in that specific body cells are attacked by the immune system. In type I diabetes, the immune system attacks and destroys insulin-secreting islet cells in the pancreas. In multiple sclerosis, the immune system attacks and destroys the myelin sheaths that normally insulate the axons of neurons and enable rapid transmission of nerve impulses.

Some relatively common autoimmune diseases are systemic, or body-wide, diseases. They include rheumatoid arthritis and systemic lupus erythematosus (SLE). In these diseases, many tissues and organs may be attacked and injured by the immune system. For example, as you can see in Figure \(\PageIndex{6}\), symptoms of SLE may involve the muscular, skeletal, integumentary, respiratory, and cardiovascular systems.

Figure \(\PageIndex{6}\): Systemic lupus erythematosus is an autoimmune disease that may cause symptoms of body-wide tissue damage. The symptoms listed here are psychological fatigue and loss of appetite, butterfly rash on the face, inflammation of pleura and pericardium, poor circulation to finger and toes, fever, photosensitivity, mouth and nose ulcers, muscle aches, and joint arthritis. Symptoms of SLE, Mikael Häggström CC0

Treatment for Autoimmune Diseases

None of these common autoimmune diseases can be cured, although they all have treatments that may help relieve symptoms and prevent some of the long-term damage they can cause. Traditional treatments for autoimmune diseases include immunosuppressive drugs to block the immune response and anti-inflammatory drugs to quell inflammation. Hormone replacement may be another option. For example, type I diabetes is treated with insulin injections because islet cells in the pancreas can no longer secrete insulin.

Immunodeficiency

Immunodeficiency occurs when the immune system is not working properly, generally because one or more components of the immune system are inactive. As a result, the immune system may be unable to fight off pathogens or cancers that a normal immune system would be able to resist. Immunodeficiency may occur for a variety of reasons.

Causes of Immunodeficiency

Dozens of rare genetic diseases can result in a defective immune system. This type of immunodeficiency is called primary immunodeficiency. One is born with one of these diseases rather than acquiring it later in life. Probably the best known of these primary immunodeficiency diseases is severe combined immunodeficiency (SCID). It is also known as “bubble boy disease” because people with this disorder are extremely vulnerable to infectious diseases, and some of them have become well known for living inside a bubble that provides a sterile environment. SCID is most often caused by an X-linked recessive mutation that disrupts normal B- and T-cell development.

Other types of immunodeficiency are not present at birth but acquired due to experiences or exposures that occur after birth. Acquired immunodeficiency is called secondary immunodeficiency because it is secondary to some other event or exposure. Secondary immunodeficiency may occur for a number of different reasons:

• The immune system naturally becomes less effective as people get older. This age-related decline, called immunosenescence, generally begins around age 50 and worsens with advancing age. Immunosenescence is why older people are generally more susceptible to disease than younger people.
• The immune system may be damaged by another disorder, such as obesity, alcoholism, or the abuse of other drugs.
• In developing countries, malnutrition is the most common cause of immune system damage and immunodeficiency. Inadequate protein intake is especially damaging to the immune system. It can lead to impaired complement system activity, phagocyte dysfunction, and reduced antibody and cytokine production.
• Surgical removal or disease of the thymus, where T lymphocytes normally mature, results in severe immunodeficiency. People without a functioning thymus are extremely susceptible to infections.
• Certain medications can suppress the immune system. This is the intended effect of immunosuppressant drugs given to people with transplanted organs so they do not reject them. In many cases, however, immunosuppression is an unwanted side effect of drugs used to treat other disorders.
• Some pathogens attack and destroy cells of the immune system. An example is a virus known as HIV, which attacks and destroys T cells.

Focus on HIV

The human immunodeficiency virus (HIV) is the most common cause of immunodeficiency in the world today, so it is the focus of the rest of this concept. It is also covered in the concept of HIV and AIDS. HIV infections of human hosts are a relatively recent phenomenon. 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 worldwide. HIV was first identified by medical researchers in 1981. Since then, HIV has killed almost 40 million people worldwide, and its economic toll has also been enormous. The hardest-hit countries are in Africa, where the virus has infected human populations the longest and where medications to control it are least available.

HIV Transmission

HIV is transmitted through direct contact of mucous membranes or body fluids such as blood, semen, or breast milk. As shown in Figure \(\PageIndex{7}\), the transmission of the virus can occur through sexual contact or the use of contaminated hypodermic needles. It can also be transmitted from an infected mother’s blood during late pregnancy or childbirth, or through breast milk after birth. In the past, HIV was also transmitted occasionally through blood transfusions. Because donated blood is now screened for HIV, the virus is no longer transmitted this way.

Figure \(\PageIndex{7}\): HIV may be transmitted in all of the ways shown here. HIV Infection, Hana Zavadska and Laura Guerin CC-BY-NC 3.0

HIV and the Immune System

HIV infects and destroys helper T cells, a type of lymphocyte that regulates the immune response. How this occurs is shown in Figure \(\PageIndex{8}\). The virus injects its own nucleic acid into a helper T cell and uses the T cell’s “machinery” to make copies of itself. In the process, the helper T cell is destroyed, and the virus copies go on to infect other helper T cells. HIV is able to evade the immune system and keep destroying helper T cells by mutating frequently, so its surface antigens keep changing, and by using the host cell’s membrane to hide its own antigens.

Figure \(\PageIndex{8}\): The human immunodeficiency virus (HIV-I) enters the T-lymphocyte, where the virus loses its outer envelope, releasing its RNA and its reverse transcriptase. The reverse transcriptase builds a complementary DNA strand from the viral RNA template. The DNA helix is inserted into the host genome. When this is transcribed by the infected cell, the new viral RNA and proteins are produced, forming new viruses that bud from the cell membrane, thus completing the virus's life cycle. AIDS life cycle illustration, NIH CC0

Acquired immunodeficiency syndrome (AIDS) may result 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. Opportunistic diseases are infections and tumors that are rare in healthy people but common in those with a damaged immune system. The diseases take advantage of the “opportunity” presented by people whose immune systems cannot fight back. Opportunistic diseases are usually the direct cause of death of people with AIDS.

Treating HIV/AIDS

For patients who have access to HIV medications, infection with the virus has ceased to be the death sentence that it once was. By 1995, combinations of drugs called “highly active antiretroviral therapy” were developed. For some patients, these drugs can reduce the amount of virus they are carrying to undetectable levels. However, some amount of virus always hides in the body’s immune cells and will multiply again if a patient stops taking the medications. Researchers are also trying to develop drugs to kill these hidden viruses. If their efforts are successful, it could bring an end to AIDS.

Feature: Human Biology in the News

EpiPens® and its sole manufacturer, pharmaceutical company Mylan, were featured in news headlines in 2016, but not for good reason. A drastic price hike in EpiPens® and Mylan’s apparent greed triggered the media outburst.

EpiPens® are auto-injectable syringes preloaded with a measured dose of epinephrine, a drug that can rapidly stop a life-threatening anaphylactic response to an allergen. Using the device is easy and does not require any special training. The injector just needs to be jammed against the thigh, whether through clothing or bare skin. Each year, doctors write millions of prescriptions for EpiPens®. Many people with severe allergies always carry two of the devices with them just in case they experience anaphylaxis, although most of them never need to use them. Other people with severe allergies have literally had their lives saved multiple times by EpiPens® when they had anaphylactic reactions. Even when the devices haven’t been used, they must be replaced each year because the epinephrine expires.

You might think that EpiPens® would be relatively inexpensive, given their life-saving potential. As recently as 2009, a two-pack of EpiPens® cost about $100. However, in just 7 years, the cost of the same two-pack of EpiPens® skyrocketed by an incredible 400 percent! By 2016, the cost was $600 or more. Mylan apparently raised the price solely to increase profits. The company also raised prices significantly on many other drugs. The price hike in EpiPens® alone was certainly profitable. In 2015, the sale of EpiPens® earned Mylan $1 billion. Mylan’s CEO took home almost $19 million the same year, which was an increase of more than 600 percent over her prior salary.

News coverage of the price hike in EpiPens® began in the summer of 2016, following a May price increase that year. Both private citizens and elected officials expressed outrage over the price increase, especially given the company's and its CEO's gluttonous profits. By late August, Mylan responded to the backlash by offering discount coupons for EpiPens®. A few days later, the company promised to introduce a cheaper, generic version of the device. Analysts quickly determined that selling a generic version would allow Mylan to make more money on the product than reducing the price of the name-brand device, which they still declined to do. By September of 2016, Mylan was under investigation for antitrust violations related to the sale of EpiPens® to public schools in New York City.

The Mylan/EpiPen® story may still be making the news. But whatever its outcome, the story has already added fuel to public and private debates about important ethical issues — such as the high costs of life-saving drugs and the huge profits of the companies that make them. What is the most recent news on EpiPens® and Mylan? If you are interested, you can check the headlines online. What are your views on the ethical issues they raise?

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