Question 1

Most B and T lymphocytes in the circulation appear as small, inactive cells, with little cytoplasm, few cytoplasmic organelles, and nuclei containing condensed inactive chromatin. Yet these cells comprise the adaptive immune response, without which individuals die in infancy. What is the explanation for this apparent dichotomy?

Accepted Answer

B and T lymphocytes are a heterogeneous population, comprised of cells that each express a unique antigen receptor. As a consequence, only a small number of B and T lymphocytes will respond to any particular pathogenic infection. The vast majority of the circulating cells will never encounter the pathogen that binds to their antigen receptor; hence these cells remain in a naive, inactive state.

Question 2

A common mechanism by which sensor cells in the host detect micro-organisms relies on the production of unique microbial components not found in the host. Propose a strategy by which a clever microbe could evade this type of response.

Accepted Answer

Sensor cells commonly recognize unique microbial components, such as bacterial lipopolysaccharide or other cell wall constituents. A clever microbe could evade this response by altering its membrane or cell wall components so that they are no longer recognized by the sensor cell receptors.

Question 3

Given the enormous heterogeneity of antigen receptors expressed on the populations of naive B and T lymphocytes, the adaptive immune response relies on a process whereby the rare lymphocyte that binds to the antigen is first induced to proliferate, before it can perform its effector function. For B cells, there is a clever mechanism that ensures that the specificity of the antibody secreted by the plasma cell will recognize the same pathogen that initially stimulated the B cell antigen receptor and induced B cell proliferation. This mechanism is:
A) The naive B cell expresses an array of different B cell antigen receptors, and randomly chooses which specificity of antibody to secrete as a plasma cell.
B) The naive B cell expresses a single specificity of B cell antigen receptor, and then up-regulates the expression of this receptor so it can bind tightly to the pathogen.
C) The plasma cell proliferates after it has finished secreting antibody to generate more plasma cells with specificity for the pathogen.
D) The plasma cell traps secreted antibody molecules in its extracellular matrix and uses these antibodies to bind to the pathogen.
E) The naive B cell expresses a membrane-bound form of the antibody as a receptor, and secretes that same antibody when it differentiates into a plasma cell.

Accepted Answer

The naive B cell expresses a membrane-bound form of the antibody as a receptor, and upon activation and differentiation into a plasma cell, it secretes a soluble form of that same antibody, ensuring specificity for the pathogen that initiated the response.

Question 4

When macrophages in a tissue encounter bacteria, they release cytokines that induce an inflammatory response. These cytokines act on other immune cells, to recruit them to the site of infection and to enhance their activities. In addition, these cytokines act on the endothelial cells of the blood vessel wall to:
A) Increase their permeability, allowing fluid and proteins to leak into the tissue
B) Solidify the tight junctions to prevent the bacteria from entering the blood
C) Proliferate, allowing the blood vessel to enlarge
D) Up-regulate microbicidal mechanisms, so they can kill bacteria
E) Secrete anti-microbial peptides

Accepted Answer

Cytokines released by macrophages during an inflammatory response act on endothelial cells of the blood vessel wall to increase their permeability. This is done in order to allow fluid and proteins to leak into the tissue, which helps with the immune response against the bacteria.

Question 5

The skin and bodily secretions provide the first line of defense against infection. One response in this category that is common during upper respiratory virus infections is:
A) Production of antibodies
B) Infiltration by white blood cells
C) Mucus production
D) Increased saliva production
E) Fever

Accepted Answer

Mucus production is a common response to upper respiratory viral infections as it helps trap and prevent further spread of the virus within the respiratory tract. Mucus also contains immune system components that can help fight off the infection. The other options listed may also occur during infections but are not specific to the first line of defense.

Question 6

The antibody protein is often depicted as an uppercase letter Y, with the two variable regions (antigen-binding domains) pointing up, and the stem consisting of the Fc region (constant domain). An analogy has been made between an antibody protein and a guided missile, with one type of antibody domain functioning as the guidance system, and the other type of domain as the 'payload.' Which antibody domain serves as the guidance system, and which as the payload? Explain your answer.

Accepted Answer

The answer of The antibody protein is often depicted as...

Question 7

Pathogenic organisms cause damage to the host by a variety of mechanisms, depending on the category of the pathogen and its mode of replication in the host. Give an example of two different types of pathogens that are unlikely to be dealt with by the same mechanism of immune protection.

Accepted Answer

The answer of Pathogenic organisms cause damage to the host...

Question 8

Innate lymphoid cells and NK cells are effector cells that respond rapidly after encountering a pathogen. Several different subsets of innate lymphoid cells exist, and each is specialized to respond to a category of pathogen (e.g., viruses, extracellular bacteria, helminthic parasites, etc). Innate lymphoid cells reside primarily in tissues such as the lungs, the lining of the gastrointestinal tract, and the skin, because these sites represent the major routes of entry of pathogens into the body.

Accepted Answer

Innate lymphoid cells and NK cells are effector cells that respond quickly to pathogens, and different subsets of innate lymphoid cells are specialized to respond to specific categories of pathogens. These cells are primarily found in tissues that represent common routes of entry for pathogens.

Question 9

Some Pattern Recognition Receptors (PRRs) recognize nucleic acids, like RNA or DNA. Since our own cells contain human RNA and DNA, the activation of innate immune pathways by these PRRs must rely on additional criteria to discriminate self from nonself. Additional criteria include everything EXCEPT:
A) The subcellular location of the RNA
B) The presence of adenosine residues in viral RNA
C) The methylation state of the DNA
D) Unique structures found on viral RNA
E) The subcellular location of the DNA

Accepted Answer

The presence of adenosine residues in viral RNA is not a criterion used to discriminate self from nonself by Pattern Recognition Receptors (PRRs), as adenosine is a common component of all RNA, both viral and host.

Question 10

Our immune system efficiently kills all categories of microbes that attempt to colonize our bodies.

Accepted Answer

Our immune system is not 100% efficient and can sometimes fail to eliminate certain microbes, leading to infections and diseases. Some pathogens can even evade detection and destruction by the immune system, making it more difficult to eliminate them.

Question 11

The clonal selection theory was first proposed in the 1950s, decades before the molecular details of B and T lymphocyte development and lymphocyte antigen recognition responses were elucidated. Nonetheless, Burnet, who proposed this theory, correctly inferred several key aspects of adaptive immune responses. One key postulate that Burnet proposed was that:
A) Cells of the innate immune system are distinct from those of the adaptive immune system.
B) Cells of the adaptive immune system are generated from a pluripotent hematopoietic stem cell that resides in the bone marrow.
C) B and T lymphocytes are closely related cells that have distinct properties from myeloid cells.
D) Circulating antibodies are generated by many different antibody-secreting cells, each of which expresses a single type of antibody on its surface as a receptor.
E) Antibodies binding to pathogens lead to efficient pathogen clearance by phagocytic cells.

Accepted Answer

Burnet's clonal selection theory primarily posits that each lymphocyte bears a single type of receptor with a unique specificity and that the binding of a foreign molecule to this receptor selects that lymphocyte for clonal expansion. This concept underlies the generation of specific antibodies by plasma cells, which are derived from B cells that have been activated by their specific antigen.

Question 12

The antigen receptor on a T cell recognizes a degraded fragment of a protein (i.e., a peptide) bound to a specialized cell surface peptide-binding receptor called an MHC molecule. One key aspect of this system is that the peptides displayed on MHC molecules can be derived from intracellular proteins. This mode of antigen recognition is particularly important in allowing the adaptive immune response to detect infections by:
A) Large helminthic parasites in the gastrointestinal tract
B) Intracellular pathogens, such as viruses and some protozoa
C) Extracellular bacteria that colonize the lungs
D) Fungi that form hyphae in the bronchial airways
E) Fungal infections in the skin epithelium

Accepted Answer

The ability to recognize peptides derived from intracellular proteins allows T cells to detect intracellular pathogens, such as viruses and some protozoa, which are able to replicate inside host cells. This mode of antigen recognition is less relevant for extracellular parasites or pathogens, which are not directly present within host cells.

Question 13

A common characteristic of a site of infection, such as a pimple on the skin, is pus. What is responsible for the white color of pus?

Accepted Answer

The answer of A common characteristic of a site of...

Question 14

Adaptive immune responses are slow to develop, taking days to weeks after exposure to reach their peak. However, these responses are more specific than innate responses, and also generate immunological memory. These latter features, which provide enhanced protection upon re-infection with the same pathogen, are the basis of:
A) Vaccines
B) Antibiotics
C) Systemic shock
D) Complement activation
E) Phagocytosis

Accepted Answer

Vaccines are developed based on the ability of adaptive immune responses to generate immunological memory, providing enhanced protection upon re-infection with the same pathogen. This is achieved by exposing the immune system to a weakened or inactivated form of the pathogen, which triggers the development of a specific immune response without causing disease.

Question 15

For cells of the innate immune system, each individual cell has multiple pattern recognition receptors, and can recognize many different pathogens. In contrast, cells of the adaptive immune system each express only a single antigen receptor, and have a single specificity for pathogen recognition.

Accepted Answer

Cells of the innate immune system, such as macrophages and dendritic cells, have multiple pattern recognition receptors (PRRs) that allow them to recognize a wide range of pathogens. In contrast, cells of the adaptive immune system, such as B cells and T cells, each express a unique antigen receptor that is specific to a single antigen, providing a highly specific response to pathogens.

Question 16

Dendritic cells are phagocytic, but also capable of ingesting large amounts of extracellular fluid and its contents, a process known as macropinocytosis. What specialized function do dendritic cells have in immunity that might account for their need to perform macropinocytosis?

Accepted Answer

The answer of Dendritic cells are phagocytic, but also capable...

Question 17

In the absence of an infection, most granulocytes (neutrophils, eosinophils, basophils) are found circulating in the blood, whereas other subsets of myeloid cells reside in tissues.

Accepted Answer

This statement is true. Granulocytes are a type of white blood cell that is characterized by granules in their cytoplasm. Neutrophils, eosinophils, and basophils are all types of granulocytes that are found circulating in the blood. However, other subsets of myeloid cells, such as monocytes and dendritic cells, can be found in tissues.

Question 18

In patients with lymphomas, the cancer cells invade the bone marrow and destroy the environment required for normal hematopoiesis. This leads to bone marrow failure, which disrupts the production of hematopoietic cell lineages. All of the following cell types would be affected by this EXCEPT:
A) Red blood cells
B) Macrophages
C) Lymphocytes
D) Endothelial cells
E) Granulocytes

Accepted Answer

Endothelial cells are not part of the hematopoietic cell lineages and thus would not be affected by bone marrow failure in lymphomas. Red blood cells, lymphocytes, macrophages, and granulocytes are all produced through hematopoiesis and would be affected by bone marrow failure.

Question 19

In the 1970s, immunologists discovered the genetic mechanism allowing a population of B cells to produce an enormous diversity of different antibodies. At the time, this discovery shocked the field of biology, as it called into question the ‘immutable’ nature of DNA, which was known to be the genetic material transmitted from generation to generation during the propagation of the species. Briefly describe this startling mechanism.

Accepted Answer

The answer of In the 1970s, immunologists discovered the genetic...

Question 20

Unlike B lymphocytes, T lymphocytes do not generate a secreted form of their antigen receptor after they are activated and proliferate. This is because the effector functions of T cells are restricted to:
A) Responses important in protozoan infections, but not other types of infections
B) Interactions with large helminthic parasites, which cannot be phagocytosed
C) Interactions with other cells, such as virus-infected cells or other immune cells
D) Responses important in mucosal surfaces (e.g., the lung), where antibodies cannot go
E) Stimulating B cells and not any other types of cells

Accepted Answer

T cells interact with other cells, such as virus-infected cells or other immune cells, through direct cell-to-cell contact mediated by their T cell receptor (TCR) and other surface molecules. They do not secrete antibodies like B cells do, but instead, they can secrete cytokines that can activate other immune cells and coordinate immune responses.

Quiz 1: Basic Concepts in Immunology