MCAT Biology : MCAT Biological Sciences

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #61 : Immune And Lymphatic Systems

Which of the following is not a component of innate immunity?

Possible Answers:

Macrophages

Plasma cells

Stomach acid

Mast cells

Correct answer:

Plasma cells

Explanation:

The innate immune system defends against pathogens, even without identifying what the pathogens are. Innate immunity includes physical and chemical barriers, such as the skin and stomach acid, which can kill some bacteria or prevent their entry into the body. Macrophages are responsible for phagocytosis of foreign pathogens without antibody mediation. Mast cells release histamine in response to infection or injury. Plasma cells, however, are mature B-cells that secrete antibodies in response to initiation of the adaptive immune response.

Example Question #3 : Adaptive And Innate Immunity

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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An antibody response is similar to an inflammatory response in that:

I. They are both innate immune responses

II. They are both most effective immediately following the first infection with a pathogen

III. They are both dependent on chemical mediators, such as cytokines, for function

Possible Answers:

I and III

I, II, and III

III only

II only

I and II

Correct answer:

III only

Explanation:

Cytokines are important immune mediators, and are components of both adaptive and innate immunity. B- and T-lymphocytes are mediators of adaptive immunity, while inflammation is a non-specific innate immune response to a pathogen. While innate immunity, such as the inflammatory response, is active immediately following an infection, adaptive immunity requires a previous exposure to the pathogen to become most effective. The body must already recognize the invading antigen in order to activate the antibody response.

Statement I is false, statement II is false, and statement III is true.

Example Question #2 : Adaptive And Innate Immunity

Duchenne Muscular Dystrophy is an X-linked recessive genetic disorder, resulting in the loss of the dystrophin protein. In healthy muscle, dystrophin localizes to the sarcolemma and helps anchor the muscle fiber to the basal lamina. The loss of this protein results in progressive muscle weakness, and eventually death.

In the muscle fibers, the effects of the disease can be exacerbated by auto-immune interference. Weakness of the sarcolemma leads to damage and tears in the membrane. The body’s immune system recognizes the damage and attempts to repair it. However, since the damage exists as a chronic condition, leukocytes begin to present the damaged protein fragments as antigens, stimulating a targeted attack on the damaged parts of the muscle fiber. The attack causes inflammation, fibrosis, and necrosis, further weakening the muscle.

Studies have shown that despite the severe pathology of the muscle fibers, the innervation of the muscle is unaffected.

Which compound is most likely responsible for initiating the inflammation response that results from the autoimmune attack?

Possible Answers:

Fibrogen

Dopamine

Fibrin

Histamine

Fibrinogen

Correct answer:

Histamine

Explanation:

Fibrin and fibrinogen are involved in wound healing and scab formation. Dopamine is a neurotransmitter in the brain. Fibrogen is not a real protein at all.

Histamine is the primary inflammatory molecule in the body and is released from mast cells during an immune response or trauma.

Example Question #4 : Adaptive And Innate Immunity

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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Which of the following constitutes a loss of innate immunity in an immunocompromised patient?

Possible Answers:

The down-regulation of an antibody response in a patient receiving immunosuppressive drugs

The loss of a thymus in a patient with Di George syndrome

The loss of pre-lymphocytes in a patient undergoing a bone marrow transplant

The loss of intact skin in a burn patient

The loss of a memory immune response in a patient who did not receive follow up vaccinations

Correct answer:

The loss of intact skin in a burn patient

Explanation:

The immune system comprises more than what you might initially expect. Intact skin is an important part of innate immunity, while all the other choices listed are more precisely components of the adaptive immune system. T-cells and B-cells are not involved in innate immunity.

Example Question #8 : Adaptive And Innate Immunity

Cryptosporidium is a genus of gastrointestinal parasite that infects the intestinal epithelium of mammals. Cryptosporidium is water-borne, and is an apicomplexan parasite. This phylum also includes Plasmodium, Babesia, and Toxoplasma. 

Apicomplexans are unique due to their apicoplast, an apical organelle that helps penetrate mammalian epithelium. In the case of cryptosporidium, there is an interaction between the surface proteins of mammalian epithelial tissue and those of the apical portion of the cryptosporidium infective stage, or oocyst. A scientist is conducting an experiment to test the hypothesis that the oocyst secretes a peptide compound that neutralizes intestinal defense cells. These defense cells are resident in the intestinal epithelium, and defend the tissue by phagocytizing the oocysts. 

She sets up the following experiment:

As the neutralizing compound was believed to be secreted by the oocyst, the scientist collected oocysts onto growth media. The oocysts were grown among intestinal epithelial cells, and then the media was collected. The media was then added to another plate where Toxoplasma gondii was growing with intestinal epithelial cells. A second plate of Toxoplasma gondii was grown with the same type of intestinal epithelium, but no oocyst-sourced media was added.

A patient is hiking through Nepal and comes down with a case of diarrhea caused by cryptosporidium. You determine that his body was fighting this infection mainly by mounting an antibody response. Where do the cells most directly responsible for this response develop?

Possible Answers:

Bone marrow

Thymus

Liver

Adrenal glands

Spleen

Correct answer:

Bone marrow

Explanation:

Antibodies are produced by B-cells, which develop in the bone marrow. T-cells develop in the thymus.

You can remember B for bone marrow, T for thymus.

Example Question #62 : Immune And Lymphatic Systems

Cryptosporidium is a genus of gastrointestinal parasite that infects the intestinal epithelium of mammals. Cryptosporidium is water-borne, and is an apicomplexan parasite. This phylum also includes Plasmodium, Babesia, and Toxoplasma. 

Apicomplexans are unique due to their apicoplast, an apical organelle that helps penetrate mammalian epithelium. In the case of cryptosporidium, there is an interaction between the surface proteins of mammalian epithelial tissue and those of the apical portion of the cryptosporidium infective stage, or oocyst. A scientist is conducting an experiment to test the hypothesis that the oocyst secretes a peptide compound that neutralizes intestinal defense cells. These defense cells are resident in the intestinal epithelium, and defend the tissue by phagocytizing the oocysts. 

She sets up the following experiment:

As the neutralizing compound was believed to be secreted by the oocyst, the scientist collected oocysts onto growth media. The oocysts were grown among intestinal epithelial cells, and then the media was collected. The media was then added to another plate where Toxoplasma gondii was growing with intestinal epithelial cells. A second plate of Toxoplasma gondii was grown with the same type of intestinal epithelium, but no oocyst-sourced media was added.

In the initial stages of a cryptosporidium infection, you can observe macrophages migrating to the area of infection. This process is called chemotaxis. What is likely to be the chemical mediator responsible for chemotaxis?

Possible Answers:

Neurotransmitters

cAMP

Cytokines and chemokines

Growth hormone

Androgens

Correct answer:

Cytokines and chemokines

Explanation:

Cytokines and chemokines are general classes of inflammatory mediators secreted by inflammatory cells like macrophages. They not only govern inflammation, but can also recruit surrounding cells via chemotaxis.

Example Question #2 : Adaptive And Innate Immunity

Identify the cells that correspond to the adaptive immune system and to the innate immune system.

Possible Answers:

Adaptive immune system—B-cells and T-cells

Innate immune system—monocytes, neutrophils, and macrophages

Adaptive immune system—T-cells and macrophages

Innate immune system—monocytes, neutrophils, and B-cells

Adaptive immune system—monocytes, neutrophils, and macrophages

Innate immune system—B-cells and T-cells

Adaptive immune system—monocytes, neutrophils, and B-cells

Innate immune system—T-cells and macrophages

Correct answer:

Adaptive immune system—B-cells and T-cells

Innate immune system—monocytes, neutrophils, and macrophages

Explanation:

B-cells and T-cells are part of the adaptive immune system, while monocytes, neutrophils, and macrophages are part of the innate immune system. Activation of the adaptive immune response requires prior exposure to a pathogen and the involvement of antibodies, while the innate immune response will occur whether a pathogen has been exposed before or not.

This list only contains a few examples of immune cells for each response, and is not exhaustive.

Example Question #11 : Adaptive And Innate Immunity

Which of the following cells is a part of the adaptive immune response?

Possible Answers:

Neutrophils

Mast cells

Natural killer (NK) cells

Cytotoxic T-cells

Correct answer:

Cytotoxic T-cells

Explanation:

An adaptive immune response only occurs once a specific antigen has been presented to a T-cell by an antigen-presenting cell, which activates a highly specific immune system response. The cells most directly involved in the adaptive immune response (for the purposes of the MCAT) are helper T-cells, cytotoxic T-cells, and B-cells.

On the other hand, our immune system also evades pathogens by non-specific, innate responses which encompass a wide range of cells and biological systems. While the other three answer choices are recognzied as "immune cells", they are not involved an adaptive immune response, making them incorrect answers.

Example Question #65 : Immune And Lymphatic Systems

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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A scientist is investigating what attracts monocytes to the site of infection, thus prompting diapedesis. He finds that a class of soluble mediators are given off by infected cells. This soluble mediator is most likely __________.

Possible Answers:

a fibroblast growth factor (FGF) protein

a chemokine protein

an sonic hedgehog (SHH) protein

a Wnt protein

a JAK-STAT protein

Correct answer:

a chemokine protein

Explanation:

Chemokines and cytokines are the main inflammatory mediators that drive a cellular response to inflammation or infection.

The JAK-STAT pathways are most linked to cell growth and division, while the hedgehog pathway (including SHH) is linked to early development. Wnt proteins are also linked to early development, but have been linked to carcinogenesis as well. Finally, fibroblast growth factor is linked with wound healing, but not with the initial immune response or recruitment.

Example Question #12 : Adaptive And Innate Immunity

Which of the following is not involved in innate immunity?

Possible Answers:

Mucous

Neutrophils

Macrophages

Plasma cells

Skin

Correct answer:

Plasma cells

Explanation:

Plasma cells are mature B-cells, and are not part of the innate immune response. B-cell and T-cells are part of the adaptive immune response.

The innate immune response is mainly comprised of physical barriers and phagocytes. Skin and mucous are examples of such physical barriers, while macrophages and neutrophils are examples of non-specific phagocytes.

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