Exam 1: Basic Concepts in Immunology

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:

T cells expressing the co-receptor CD8 are generally cytotoxic cells, with an important function in eliminating virus infections that can occur in many different cell types and tissues. In contrast, CD4 T cells directly interact with a very restricted set of cells, such as dendritic cells, macrophages, and B cells. Describe one important mechanism that accounts for this division of labor between CD8 and CD4 T cells.

The mucosal tissues of the body have their own unique set of immune structures that function as sites for initiating adaptive immune responses. The necessity for mucosa-associated lymphoid tissues to have unique cell types (M cells) and structures is because:

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.

The spleen is a secondary lymphoid organ that performs several functions. In addition to its role as a site for initiating adaptive immune responses, the spleen is important in removing dead or damaged red blood cells from the circulation. Its immune function is important because blood-borne pathogens will not be transported to draining lymph nodes via the lymph fluid.

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

An infant with recurrent bacterial and fungal infections is suspected to have an immunodeficiency disease. Within two days after exposure to a pathogen, the organisms have proliferated to dangerous levels requiring immediate systemic antibiotic treatment. It is unlikely that this infant has a defect in B or T lymphocyte responses to the infection because:

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.

T_H1, T_H2, T_H17, and T follicular helper (T_FH) cells represent four different subsets of CD4 effector cells. Each of these subsets produces a distinct set of cytokines when stimulated, that in turn, act to mobilize distinct immune effector mechanisms. While T_H1, T_H2, and T_H17 cells recruit and activate innate immune cells, T_FH cells act to amplify the adaptive immune response.

Naive B and T lymphocytes are small, quiescent cells with little cytoplasm and low metabolic activity. Yet within hours after being activated following encounter with their antigen, these cells enlarge and up-regulate many biosynthetic and metabolic pathways. Approximately one day later, the cells begin dividing, and for several days they are the most rapidly dividing cells in the body, undergoing 2-4 rounds of cell division every day. In order to maintain this phenomenal rate of cell division, lymphoblasts must:

For each of the panels A–D in Figure , identify the most likely component(s) of the immune response indicated by the red arrow, and briefly describe your reasoning.

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.

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?

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:

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:

The effector activities important in eliminating infectious organisms from our bodies can be categorized into four different groups: cytotoxicity, intracellular immunity, mucosal and barrier immunity, and extracellular immunity. Briefly describe why the immune system requires four different effector modules for maximum protection.

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.

The immune system uses several types of effector modules to protect us against different categories of pathogens. Four major types of pathogens are shown in Figure . a) Which of these categories might be effectively eliminated by innate immune responses that include antimicrobial peptides and phagocytic cells such as neutrophils and macrophages? Explain your answer. b) Which of these categories of pathogenic organisms might be most effectively dealt with by antibodies, if the innate response is insufficient for their eradication? c) Which of these categories of pathogenic organisms would require T lymphocyte responses for their elimination?

One major difference between the innate and adaptive immune responses is in the mechanism by which pathogens are recognized. Innate immune cells use pattern recognition receptors (PRRs) to recognize conserved determinants shared by all the members of a category of pathogens, whereas adaptive immune cells (B and T lymphocytes) have highly specific antigen receptors. a) Which of the patterns of receptor expression in Figure represent innate immune cells? b) Which of the patterns of receptor expression represent B and T lymphocytes? c) Following an infection, how does the population of innate cells change? Starting with the cartoon representing your answer to part (a), draw the population present at one week post-infection. d) Following infection, how does the population of B and T lymphocytes change? Starting with the cartoon representing your answer to part (b), draw the population present at one week post-infection.

When complement proteins are covalently deposited onto the surface of a bacterium, this can sometimes lead to direct lysis of the bacterium. However, more commonly, the deposition of complement proteins onto the bacterial surface does not directly harm the bacterium. Instead, these complement proteins aid in bacterial elimination by: