Clinical Microbiology HW#9

A Little Sting with a Big Effect

Sally is having a great time camping with her friends. They’ve had nice weather, seen a variety of wildlife, and enjoyed many fun nights around the campfire. On their last night in camp, they’re all out gathering firewood when Sally gets stung by a couple of wasps after stumbling onto a ground nest. The initial stings hurt, but Sally isn’t too worried because she’s been stung before and has never had an adverse reaction. After a few minutes, however, Sally notices that her skin feels a tingly and itchy. Her friends recommend that she sit down and take it easy for a little while. What Sally doesn’t realize yet is that she is experiencing the early stages of a hypersensitivity reaction!

1. When the wasps stung Sally, the injection of venom initiated a type I hypersensitivity reaction called anaphylaxis. Which of the following best describes a hypersensitivity reaction?

a reaction that occurs in a sensitized individual resulting in tissue damage rather than immunity

**Sally didn’t realize it, but the stings that she received prior to today were actually sensitizing events. Her body generated antibodies against wasp venom, including the antibodies that are going to be involved in mediating the hypersensitivity reaction. When the venom entered her body this time, the antibodies initiated a series of events that are going to result in a harmful, not protective, outcome.

The anaphylactic reaction that Sally is experiencing differs from a normal immune response not only in its outcome but also in some of the specific molecules and mediators that are involved.

2. Which of the following mediators are involved in anaphylactic reactions?

Basophils ; IgE ; mast cells

**During anaphylaxis, mast cells, basophils, and IgE molecules initiate a response in the body that has negative effects for the host. Although all of these are parts of the immune system and are designed to protect the host, in anaphylaxis their activation doesn’t help to protect the host.

While sitting at camp, Sally notices that her itching has started to worsen, and, moreover, hives have started forming all over her body. She also realizes that her skin is red and hot.

3. The hives that Sally is experiencing are a result of an anaphylactic reaction. This is a multistep reaction resulting from the interaction of the wasp venom and Sally’s immune system. Each of the statements below is a step in the anaphylactic response.

1. IgE molecules bind to mast cells and basophils.
2. Antigen molecules cross-link adjacent IgE molecules.
3. Degranulation occurs.
4. Chemical mediators such as histamine are released.
5. Capillary permeability and smooth muscle contractions increase.
6. Neutrophils and eosinophils move to the site.

**As the wasp venom enters Sally’s body, it binds to IgE molecules on the surface of mast cells and basophils. Molecules of venom act to cross-link adjacent IgE molecules, leading to a massive release of chemical mediators contained within the cells. In addition, the cells are also stimulated to synthesize and release other mediators, such as leukotrienes and prostaglandins. These molecules lead to physiological changes, including increased mucus secretion and blood vessel permeability. In addition, these chemicals act as chemoattractants, signaling neutrophils and other immune cells to migrate to the site.

Anaphylactic reactions are described as being either localized or systemic. The hives, itching, and redness all over Sally’s body indicate that she is having a systemic anaphylactic reaction to the wasp venom.

4. Which of the following is true regarding systemic anaphylactic reactions?

-A systemic reaction can be fatal in only a few minutes.
-Systemic reactions are commonly associated with injected antigens.
-Systemic reactions can result in a dramatic decrease in blood pressure.

**The injection of venom into Sally’s body provides a rapid route for distribution throughout her system. The effects can be seen all over her body, hence the appearance of hives. Sally is lucky that her symptoms are limited to the hives and itching—many cases of systemic anaphylaxis involve significant changes in blood pressure (which can lead to shock) and respiration.

Sally and her friends are a little freaked out by the rapid appearance of hives, the redness, and the itching. Sally’s friend Becky has some antihistamines in her first-aid kit and tells Sally to take them immediately.

5. Predict which of the following outcomes would result from taking an antihistamine.

-decreased histamine-based stimulation of cells
-decreased mucus secretions
-decreased permeability of blood vessels

**The antihistamines that Sally took actually work as a receptor antagonist. This means that the antihistamine blocks the ability of histamine to bind to its targets, thereby blocking the cellular effects of histamine. By this action, the physiological impact of histamine should begin to decrease, and Sally’s symptoms should begin to subside.

To everyone’s relief, about 30 minutes after Sally takes the antihistamine her itching begins to subside. A little later, although the hives are still there, the amount of redness has decreased as well. Sally ends up getting quite sleepy (a common side effect of antihistamines) and turns in early. The next morning, some hives still remain, but Sally feels almost back to normal for the group’s hike out of the woods. After returning home, Sally decides to follow up with her primary care physician. Sally’s doctor suggests that she undergo a desensitization program to prevent future anaphylaxis reactions against wasp venom.

6. Sally decides to go through the desensitization program recommended by her doctor. What is the overall purpose/goal of this process?

Repeated exposure to wasp venom antigen will increase the amount of IgG produced by Sally’s immune system.

7. The IgG molecules that are produced during desensitization function as blocking antibodies. These antibodies are extremely effective in providing protection from a hypersensitivity reaction. Why are blocking IgG antibodies so effective at protecting Sally from another anaphylactic response to wasp venom?

The blocking IgG antibodies “outnumber” the IgE antibodies and bind to wasp venom before the IgE antibodies can bind.

**During a desensitization program, Sally will be injected with increasing doses of wasp venom over several weeks to months. The goal is to build the level of wasp venom-specific IgG circulating in Sally’s system. If the desensitization program is successful, the next time that Sally is stung by a wasp, the IgG molecules will bind to the venom, thereby preventing (or limiting) the amount of venom antigen that binds to IgE present on mast cells and basophils.

Chapter 19 Reading Questions

1. Assuming Rh compatibility is present, which of the following individuals would be able to receive donor blood from any of the four blood types (A, B, AB, and O)?

AB

2. Which type of hypersensitivity is allergic contact dermatitis?

Type IV reaction

3. Which of the following best describes graft-versus-host disease?

Immune cells in transplanted bone marrow attack the cells of the host.

Chapter 19

1. All of the following are true of hypersensitivity EXCEPT

it occurs when an individual is exposed to an allergen for the first time.

2. The chemical mediators of anaphylaxis are

found in basophils and mast cells

3. Which antibodies will be in the serum of a person with blood type B, Rh⁺?

Anti-A

4. Which blood transfusions in the table are incompatible?

Donor	Recipient
1. AB, Rh-	AB, Rh+
2. A, Rh+	A, Rh-
3. A, Rh+	O, Rh+
4. B, Rh-	B, Rh+
5. B, Rh+	A, Rh+

2, 3, and 5

5. Hemolytic disease of the newborn can result from an

Rh- mother with an Rh+ fetus

6. Treatment with certain drugs to reduce transplant rejection can cause

immunosuppression.

7. Which of the following is a possible outcome of an HIV infection?

All of the answers are correct.

**There may be persistent yeast infections.

Infection may initially be asymptomatic.

Virions may remain latent.

The disease does not progress to AIDS.

8. Immunodeficiencies are a result of all of the following EXCEPT

autoimmune diseases.

9. Symptoms of delayed cell-mediated reactions are due to

Cytokines

10. Individuals who do not express CCR5 are highly resistant to infection by HIV.

TRUE

11. Vaccines against HIV have proven to be very effective in halting the spread of disease.

FALSE

12. Autoimmune diseases occur when the immune system is unable to discriminate self from nonself.

TRUE

13. Cancer is similar to an infectious disease in that it is due to a failure of the body’s defenses.

TRUE