MCAT Biology : Antibodies and Antigens

Study concepts, example questions & explanations for MCAT Biology

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Example Question #82 : Immune And Lymphatic Systems

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.

 

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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 __________.

Possible Answers:

CCR5

interleukin-2

a T-cell receptor

CD28 ligand

a major histocompatibility complex

Correct answer:

a major histocompatibility complex

Explanation:

Major histocompatibility complex (MHC) class I is found on all nucleated cell types, while MHC class II is limited to antigen-presenting cells, such as dendritic cells. MHC class I presents foreign antigens from intracellular parasites to CD8+ T-cells in an effort to demonstrate infection and initiate cell killing.

C-C chemokine receptor type 5 (CCR5) is a specific chemokine receptor on the surface of T-cells, and is involved in cell recruitment to initiate the immune response. CD28 ligand is expressed by antigen-presenting cells and binds to T-cell receptors to activate T-cells. Interleukin-2 (IL-2) is a cytokine secreted into the blood to help activate the T-cell immune response.

Example Question #83 : Immune And Lymphatic Systems

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

Possible Answers:

Rh-negative mother and Rh-positive fetus

Rh-positive mother and Rh-negative fetus

Rh-negative mother and Rh-negative fetus

Rh-positive mother and Rh-positive fetus

Correct answer:

Rh-negative mother and Rh-positive fetus

Explanation:

Rh factors are surface proteins found on red blood cells. An Rh-negative mother can be exposed to Rh-positive blood from the fetus in her first pregnancy. Without administration of Rh(o) D immunoglobulin during the delivery of her first baby, the mother can develop antibodies to Rh so that during her second pregnancy, the maternal antibodies will cross the placenta and attack the red blood cells of the fetus if it is Rh-positive. The attack on fetal red blood cells will require blood transfusions for the fetus.

Example Question #61 : Immune System

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.

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

Possible Answers:

They function mainly by opening holes in the bacterial cell membranes

They are produced by T-cells

They are produced by dendritic cells

They are lipid molecules

They are produced by B-cells

Correct answer:

They are produced by B-cells

Explanation:

The main function of bone marrow derived B-cells is to produce antibodies. T-cells are involved in helping the B-cell response, as well as participating in cell-mediated cytotoxicity.

Example Question #62 : Immune System

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.

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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?

Possible Answers:

They are produced by B-cells

They are a key component of innate immunity

They are produced by T-cells

They are principally carbohydrate

They are of a single isotype

Correct answer:

They are produced by B-cells

Explanation:

Antibodies are a key component of adaptive immunity and occur in five main isotypes: IgA, IgD, IgE, IgG, and IgM. They are mainly composed of protein. Antibodies are produced by B-cells in response to a known antigen.

Example Question #63 : Immune System

 

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?

Possible Answers:

IgE

IgG

IgM

IgD

IgA

Correct answer:

IgA

Explanation:

Because V. cholerae enters the body through the digestive tract, the best antibody that can prevent infection would be one that is secreted into the small intestine. IgA is a secretory antibody that can be secreted by the cells lining the small intestine, into the small intestine lumen.

The other antibodies (IgG, IgM, IgD, and IgE) cannot be secreted out of the body and would only be effective once the V. cholerae has entered the body; therefore, IgA is the correct answer.

Example Question #64 : Immune System

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

Possible Answers:

The patient can receive AB blood

Both of the patient's parents must have type A blood

The patient creates A antibodies

The patient can receive O negative blood

Correct answer:

The patient can receive O negative blood

Explanation:

When looking at blood types, remember that the allele for blood type represents the type of antigen presented on the person's red blood cells. The positive or negative sign is indicative of whether or not the person makes Rh factors. If a person is negative, they create antibodies to positive Rh factors. If a person is missing an allele in their blood type, they will make antibodies for that particular antigen.

The patient in question will have antigens for type A, and antibodies against type B and Rh factor.

Type O negative blood means that there are no antigens on the red blood cells, meaning that a person with type A negative blood can receive type O negative blood. O negative blood is widely considered the "universal donor" type because it lacks any antigens that may react with antibodies in a recipient's blood. We cannot draw conclusions about the patient's parents; we know that one parent carried the A allele, but they could have been AB and the second parent could have been type O.

Example Question #65 : Immune System

Which part of the antibody recognizes the antigen?

Possible Answers:

The variable and constant regions of the heavy chains

The variable regions of the heavy and light chain

The variable regions of the light chains

The constant regions of the heavy and light chain

Correct answer:

The variable regions of the heavy and light chain

Explanation:

The antibody has two light chains and two heavy chains, each with a constant and a variable region. The variable regions of each chain are randomized during cell proliferation and recognize different antigens. This provides diversity of recognition for a better immune system.

Example Question #66 : Immune System

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.

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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

Possible Answers:

I, only

I and II

II and III

II, only

III, only

Correct answer:

II, only

Explanation:

B-cells are the only cells to produce antibodies, which then target pathogens (or normal tissue, in autoimmune disease) for phagocytosis or cell-killing via other immune pathways. T-cells mediate the adaptive immune response and activation of B-cells, but do not produce antibodies. Macrophages help to phagocytose foreign particulates and pathogens, and can react to the antigens or antibodies attached to a foreign pathogen, but do not produce antibodies.

Example Question #67 : Immune System

Which of the following cell types secretes antibodies?

Possible Answers:

Natural killer cells

B-cells

Neutrophils

Macrophages

T-cells

Correct answer:

B-cells

Explanation:

B-cells are responsible for the humoral immune response, which is the production of antibodies in response to a specific antigen. T-cells mediate the adaptive immune response by helping to activate B-cells (helper T-cells) and attacking foreign pathogens (cytotoxic T-cells). Natural killer cells are part of the innate immune response, and kill infected or damaged cells. Macrophages and neutrophils are phagocytes and help to attack and digest pathogens.

Example Question #68 : Immune System

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

Possible Answers:

Bacteria

Fungi

Viruses

Parasites

Correct answer:

Viruses

Explanation:

The "self" antigens that prevent T-cells from attacking the body are called MHC molecules. These molecules come in two classes: class I and class II. RNA, DNA, and proteins in viruses are displayed after the virus is ingested by an antigen-presenting cell using MHC class I molecules. The antigen-presenting cell reports the MHC molecule corresponding to the virus to a helper T-cells. Once the helper T-cells see this unrecognized part of the virus, and detect it as different from "self," they can initiate the adaptive immune response.

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