Immune Responses to Pathogens Flashcards
Do dendritic cells participate in innate or adaptive immune responses?
Both.
Dendritic cells that participate in part of the innate responses are responsible for triggering the adaptive immune responses.
Dendritic cells present peptides derived from pathogens to naïve T cells to alert the naïve T cells of an infection, and then provide a co-stimulatory signal to fully activate the naïve T cells and initiate the adaptive immune response.
Is adaptive immunity independent of innate immunity?
Adaptive immunity is much more effective than, but not independent of innate immunity.
What is an extracellular bacterial infection?
Those infections where the bacteria do not invade the cells of the host.
What is the most appropriate form of an adaptive immune-response for extracellular bacterial infection?
Production of antibodies would be the most appropriate form of an adaptive immune response.
The antibodies would function to neutralize the pathogen, or opsonize the pathogen to make phagocytosis more efficient, or activate complement to kill the pathogen.
Describe the immune response prior to adaptive immunity activation if you cut your finger on a piece of glass contaminated with bacteria.
The first line of defense (intact skin) has been breached.
At the site of tissue damage, the mast cells that reside in the tissues release histamine that increase the blood flow to the area, resulting in inflammation (redness, swelling, increased local temperature).
During the early innate responses, complement proteins from the blood enter the tissues and can kill many types of bacteria via the alternate pathway.
Resident macrophages recognize pathogen-associated molecular patterns (PAMPs) with their pattern recognition receptors (PRRs including Toll-like receptors, TLRs) and phagocytose bacteria, and produce alarm cytokines to start the induced innate responses.
The resident macrophages release substances that help recruit neutrophils and monocytes to the site of infection.
During the induced innate responses, neutrophils arrive at the site of infection within minutes and immediately begin ingesting bacteria by phagocytosis.
After several hours, the monocytes that have entered the tissues differentiate into macrophages. Macrophages are very efficient phagocytic cells and also secrete bactericidal substances.
However, if there are large numbers of bacteria, the bacteria could overwhelm the innate responses.
The bacteria could multiply and spread to other sites in the body.
The adaptive immune system would be activated as a last line of defense.
What do complement proteins do during the early innate response?
Complement proteins from the blood enter the tissues and can kill many types of bacteria via the alternate pathway
How do resident macrophages start the induced innate responses?
They recognize pathogen-associated molecular patterns (PAMPs) with their pattern recognition receptors (PRRs including Toll-like receptors, TLRs), phagocytose bacteria, and produce alarm cytokines.
What do resident macrophages recruit?
The resident macrophages release substances that help recruit neutrophils and monocytes to the site of infection
During the induced innate responses, how soon do neutrophils arrive at the site of infection?
Within minutes and immediately begin ingesting bacteria by phagocytosis.
How many hours do monocytes take to differentiate into macrophages during the induced innate responses?
After several hours, the monocytes that have entered the tissues differentiate into macrophages.
Macrophages are very efficient phagocytic cells and also secrete bactericidal substances.
Describe why dendritic cells are one of the most important links between innate and adaptive immunity.
Immature dendritic cells in the skin engulf the bacteria, process (digest) it and display its antigenic peptides on MHC class II proteins on the cell surface.
The dendritic cells migrates from the skin to a nearby lymph node where they “presents” the antigenic peptide fragments to a T helper cell.
They take a “snapshot” of what is happening on the frontlines of the infection and bring the information to the lymph node to activate the specific lymphocytes (T cells, B cells) that will be useful in fighting that particular pathogen.
The pathogen itself may also enter the lymphatic system and be carried to nearby lymph nodes.

What cells recognize antigenic determinants on intact pathogens?
In the lymph nodes are B cells with B cell receptors (BCRs) that can recognize antigenic determinants on the unprocessed and intact pathogens.
Describe the activation of a T helper cell.
The T cell receptor (TCR) of the T helper cell binds to the MHC class II-peptide complex on the dendritic cell.
The CD3 complex of the T helper cell sends a signal to the nucleus (“signal 1” of T cell activation).
The CD28 protein on the T cell interacts with the B7 co-stimulatory protein on the dendritic cell (“signal 2” of T cell activation).
The T helper cell secretes IL-2 and binds it (“signal 3” of T cell activation) triggering proliferation and differentiation into effector cells and memory cells.
The T helper cell starts to express CD40L on its cell surface.
How does a pathogen travel to a lymph node?
The pathogen is pushed from the tissue and into the lymphatic vessel by blood pressure, where it eventually enters a lymph node.
Describe T-dependent B cell activation.
Bacteria bind to and cross-link the BCR resulting in a signaling cascade initiated by Igα/Igβ co-stimulatory proteins (“signal 1” of B cell activation).
The B cell starts to express the co-stimulatory B7 proteins on its cell surface.
The antigen is internalized and processed inside an endosome so that peptide fragments can be displayed on MHC class II proteins.
The TCR of the T helper cell binds to the MHC class II-peptide complex on the B cell, and the CD28 of the T cell binds to the B7 of the B cell.
The CD40 protein on the B cell binds to the CD40L of the T helper cell (“signal 2” of B cell activation), and the T helper cell provides the B cell with cytokines (“signal 3” of B cell activation) resulting in the complete activation of the B cell.
Cytokines secreted by the activated T helper cell also cause the activated B cell to divide, class switch to a different isotype of antibody and differentiate into plasma cells or memory B cells.
Plasma cells increase their production of ribosomes and endoplasmic reticulum in preparation for becoming “antibody factories.”
Memory B cells differentiate are important for future responses if there is a re-infection with the same pathogen.
Activated T helper cells and B cells divide many times. What do they differentiate into?
The activated T helper cells and B cells divide many times.
The lymphocytes differentiate into effector cells and memory cells.
The large increase in the number of T and B cells increases the size of the lymph node and results in “swollen glands.”
Why can you have antibodies in your blood that recognize antigens that you encountered many years ago?
The plasma cells generated during a T cell-dependent antibody response can migrate to the bone marrow and produce specific antibodies for long periods of time (many years). This explains why you may have antibodies in your blood that recognize antigens that you encountered many years ago.
What type of antibody do plasma cells produce during a T cell-independent response?
The plasma cells generated during a T cell-independent response are short-lived and produce only low affinity IgM antibodies.
How long does the primary response take for production of antibodies?
After approximately 5–7 days (for a primary response), the plasma cells start making and secreting antibodies.
Antibodies secreted by the plasma cells enter the blood and travel all over the body including the site of infection. How do the antibodies function?
The antibodies will bind to the bacteria and both neutralize it (i.e., prevent it from attaching to surfaces, IgM and IgG), opsonize it to improve the efficiency of phagocytosis (IgG), and activate complement to kill the bacteria (IgM and IgG).
Antibodies secreted into mucus will neutralize the bacteria (IgA).
What are toxins?
Some pathogens cause damage to the host by secreting proteins called toxins.
Toxins act on specific cell types and impair the function of the cell or kill the cell.
Proteins, such as toxins, that are foreign to the host can also elicit immune responses.
Describe the immune response that results after administration of tetanus toxoid (a toxin that is modified so that it is biologically non-functional).
The antibody response to a foreign protein would develop.
Antibodies generated would bind to tetanus toxin and neutralize it, by preventing it from binding to the person’s cells.
What response does the body have to extracellular bacterial infection?
Antibody-mediated response
After a dendritic cell has engulfed a bacteria, what do they do with the information?
Dendritic cells that engulf bacteria carry information from the site of infection to the lymph node to activate T helper cells.








