Hypersensitivity reactions occur when body tissues are affected by an abnormal immune reaction. The result is damage to normal tissues and clinical illness. A peanut allergy is an example of a hypersensitivity reaction, but there are three additional broad classes.

One class involves the abnormal production or deposition of antibodies. Antibodies are B-cell derived molecules that normally adhere to pathogens, rendering them unable to continue an infection. When antibodies are produced against normal tissues, however, disease can result. Figure 1 depicts a schematic structure of an antibody.

Antibodies can be divided into two peptide chains: heavy and light. Heavy chains form the backbone of the antibody, and are attached to light chains via covalent bonding. Each heavy and light chain is then further divided into constant and variable regions. Variable regions exhibit molecular variety, generating a unique chemical identity for each antibody. These unique patterns help guarantee that the body can produce antibodies to recognize many possible molecular patterns on invading pathogens.

Unlike B-cells, T-cells do not make antibodies. T-cells are important in the execution of cytotoxic immunity, such as neutralizing virus-infected cells. A scientist is studying the T-cell response in a mammal, and finds that his CD8⁺ T-cells are interacting with a surface protein found on many different types of cells in his model organism. This protein is most likely __________.

Which of the following combinations might yield the necessity of blood transfusion for a new born baby?

Sexually transmitted diseases are a common problem among young people in the United States. One of the more common diseases is caused by the bacterium Neisseria gonorrhoeae, which leads to inflammation and purulent discharge in the male and female reproductive tracts.

The bacterium has a number of systems to evade host defenses. Upon infection, it uses pili to adhere to host epithelium. The bacterium also uses an enzyme, gonococcal sialyltransferase, to transfer a sialyic acid residue to a gonococcal surface lipooligosaccharide (LOS). A depiction of this can be seen in Figure 1. The sialyic acid residue mimics the protective capsule found on other bacterial species.

Once infection is established, Neisseria preferentially infects columnar epithelial cells in the female reproductive tract, and leads to a loss of cilia on these cells. Damage to the reproductive tract can result in pelvic inflammatory disease, which can complicate pregnancies later in the life of the woman.

In an immune response to an organism like Neisseria, humans will often make use of antbodies. What is true of antibodies?

One component of the immune system is the neutrophil, a professional phagocyte that consumes invading cells. The neutrophil is ferried to the site of infection via the blood as pre-neutrophils, or monocytes, ready to differentiate as needed to defend their host.

In order to leave the blood and migrate to the tissues, where infection is active, the monocyte undergoes a process called diapedesis. Diapedesis is a process of extravasation, where the monocyte leaves the circulation by moving in between endothelial cells, enters the tissue, and matures into a neutrophil.

Diapedesis is mediated by a class of proteins called selectins, present on the monocyte membrane and the endothelium. These selectins interact, attract the monocyte to the endothelium, and allow the monocytes to roll along the endothelium until they are able to complete diapedesis by leaving the vasculature and entering the tissues.

The image below shows monocytes moving in the blood vessel, "rolling" along the vessel wall, and eventually leaving the vessel to migrate to the site of infection.

Neutrophils are able to respond to the constant region of antibodies that coat foreign invaders. The neutrophil recognizes these antibodies, and ingests the pathogen they coated. Which of the following is true of antibodies?

Cholera is a disease caused by vibrio cholerae, a bacteria which enters the body through the digestive tract. The bacteria is absorbed by the small intestine and enters the blood stream. Which of the following antibodies would be most effective at preventing cholera?

A patient is admitted to the hospital needing a blood transfusion. The patient has type A negative blood. Which of the following is true?

Which part of the antibody recognizes the antigen?

Type 1 diabetes is a well-understood autoimmune disease. Autoimmune diseases result from an immune system-mediated attack on one’s own body tissues. In normal development, an organ called the thymus introduces immune cells to the body’s normal proteins. This process is called negative selection, as those immune cells that recognize normal proteins are deleted. If cells evade this process, those that recognize normal proteins enter into circulation, where they can attack body tissues. The thymus is also important for activating T-cells that recognize foreign proteins.

As the figure below shows, immune cells typically originate in the bone marrow. Some immune cells, called T-cells, then go to the thymus for negative selection. Those that survive negative selection, enter into general circulation to fight infection. Other cells, called B-cells, directly enter general circulation from the bone marrow. It is a breakdown in this carefully orchestrated process that leads to autoimmune disease, such as type 1 diabetes.

There are many ways that the body's immune system can attack its own tissues in autoimmune disease. A scientist discovers that in type 1 diabetes, antibodies play a key role in attracting lymphocytes to normal tissue, which is then damaged or destroyed. What kinds of cells typically produce antibodies?

I. T-cells

II. B-cells

III. Macrophages

Which of the following cell types secretes antibodies?

Major histocompatibility complex (MHC) I molecules primarily display antigens derived from what type of pathogen?