Intro to host immune responses

Question 1

What are the characteristics of innate immunity?

Answer 1

1. pre-formed
2. active
3. Does NOT have immunological memory
4. does NOT improve over time
5. non-specific (first like of defense)
   - –> often unsuccessful in controlling infection so adaptive immunity kicks in

Question 2

What are the components of innate immunity?

Answer 2

1. barriers (skin, mucous)
2. cells: neutrophils, monocytes, macrophages, dendritic cells, natural killer cells (non-phagocytic)
3. antimicrobial peptides and proteins (ex: complement and C-reactive proteins)

Question 3

What are the characteristics of adaptive immunity?

Answer 3

1. slow onset
2. highly effective, once activated
3. improves over time
4. DOES have immunological memory
5. highly specific

Question 4

What are the components of adaptive immunity and what do these produce?

Answer 4

1. B lymphocytes produce antibodies (immunoglobin (Ig)). differentiate in Bone marrow. the antibodies produced by B cells bind to the antigens of microbes and neutralize their function.
2. T lymphocytes produce cytokines (interleukin (IL)). differentiate in Thymus. Cytokines do NOT bind to antigens but modulate the innate and adaptive immunities.

Question 5

What are characteristics of neutrophils?

Answer 5

1. multi-lobed nuclei
2. phagocytose organisms and kill them intracellularly
3. only live for 1 day and then replaced by new cells from bone marrow
4. have granules
5. protect against acute infection

Question 6

What are characteristics on monocytes? What can they differentiate into?

Answer 6

kidney-shaped phagocytic cells that are more efficient in phagocytosis than neutrophils and can live for 2 days in circulation. When monocytes migrate to organs they differentiate into macrophages which can live for months to years. macrophages have the best phagocytic abilities

Question 7

What are considered the MOST efficient antigen presenting cells? How do they present antigens?

Answer 7

dendritic cells internalize complex protein antigens (which cannot be recognized by T cells in their native state) and break them down into simple antigen peptides. these simple antigen peptides are then conjugated with MHCs (either class 1 or 2), presenting a antigen form on the cell surface that is recognizable to the T cells, initiating an adaptive immune response

macrophages can also participate in antigen presentation in the same manner

Question 8

Describe the classical pathway of complement activation

Answer 8

antigen-antibody reactions activate C1 followed by C4 and C2. This C142 complex activates C3 which results in the activation of C5, C6, C7, C8, and C9, collectively called the membrane attack complex. This complex then binds to bacterial cell surface causing the lysis of the bacteria

Question 9

What can lead to complement activation?

Answer 9

1. antigen-antibody reactions
2. Infection with Gram negative bacteria causing release of lipopolysaccharide (LPS)
3. Release of mannose (carbohydrate) during fungal or gram positive bacterial infection

Question 10

Describe the alternative pathway of complement activation

Answer 10

infections with gram negative bacteria results in the release of LPS. LPS directly activated C3 followed by an activation of C5-9 and lysis of the bacterial cell

Question 11

What is opsonisation?

Answer 11

phagocytic cells have receptors for complement and antibodies on their cell surfaces. when the bacterial surface is coated with complement or antibodies (opsonins), then the complement on the bacterial cell surface will bind to the corresponding receptors on the macrophage cell surface, allowing an easier and enhanced uptake of bacteria.

Question 12

What activates Natural killer cells to induce lysis in target cells? How do natural killer cells induce cell lysis?

Answer 12

Down-regulation of MHC class 1 molecules on leukocyte cell surfaces (normally caused by cancerous or infected cells) will stimulate the NK cells to cause the target cells to lyse. NK cells induce lysis by producing antimicrobial peptides, such as perforin (creates pores in the cell surface of target cells) and granulysin (enters and kills the target cells and the intracellular microbe).

Question 13

How are gram negative bacterial infections recognized as non-self?

Answer 13

LPS is released during a gram negative bacterial infection. LPS binds to LPS binding protein (from the host) which transfers LPS to CD14 which is co-expressed with TLR-4 (toll like receptor 4) on macrophages activating it. Active TLR-4 leads to production of pro-inflammatory cytokines (IL-1, IL-6, and TNF-a) which cause inflammation. These cytokines also induce liver cells to produce C-reactive proteins to attract cells to the infection, further leading to inflammation and cell death.

Question 14

What receptor mediates reactions in gram positive infections?

Answer 14

when gram positive infections occur, peptidoglycan and LTA (lipeteichoic acid) bind to TLR-2 to cause inflammation

Question 15

What is a hapten?

Answer 15

an antigen that binds to an antibody but has too low of a molecular weight to induce an immune response

Question 16

What antigen do all T cells express? What antigen do only helper T cells express and what is its function?

Answer 16

all T cells express CD3 antigen. Helper T cells express CD 4 antigen in addition to CD3. Helper T cells recognize antigens displayed on cell surface of dendritic cells in association with MHC class 2. they produce interferon-gamma that activates macrophages and neutrophils to control intracellular infection, produce IL-4, and IL-5 which activate B cells to produce antibodies, and produce IL-2 that helps with the proliferation of T cells.

Question 17

What antigen do cytotoxic T cells express? in association with chichi MHC class molecules? and how do they function?

Answer 17

CD8 antigen
MHC class 1 molecules
these cells destroy virus-infected cells by producing perforin and granulysin which will lyse the infected cell, killing both the virus and the cell.

Question 18

What cell types participate in both innate and adaptive immunity?

Answer 18

macrophages and dendritic cells

Question 19

Which antibody is important in allergic reactions?

Answer 19

IgE

Question 20

How do B cells respond to T cell independent (non-protein) antigens?

Answer 20

LPS released from gram negative bacteria binds to IgD on B cells and will directly activate B cells to produce IgM

Question 21

How do B cells respond to T cell dependent (protein) antigens?

Answer 21

antigen presented with MHC class 2 molecules will be recognized by CD4+ (helper) T cells, resulting in the production of B cell growth factor ( IL-4) and B cell stimulating factor (IL-5), inducing B cells to produce antibodies that belong to any class (IgG, IgE, IgA, IgM) –> “class switching”