Immune Responses

Question 1

IgG

Answer 1

Main antibody in 2° response to an antigen. Most abundant isotype in serum. Fixes complement, opsonizes bacteria, neutralizes bacterial toxins and viruses. Only isotype that crosses the placenta (provides infants with passive immunity).

Question 2

IgA

Answer 2

Prevents attachment of bacteria and viruses to mucous membranes; does not fix complement. Monomer (in circulation) or dimer (with J chain when secreted). Crosses epithelial cells by transcytosis. Produced in GI tract (eg, by Peyer patches) and protects against gut infections (eg, Giardia). Most produced antibody overall, but has lower serum concentrations. Released into secretions (tears, saliva, mucus) and breast milk. Picks up secretory component from epithelial cells, which protects the Fc portion from luminal proteases

Question 3

IgM

Answer 3

Produced in the 1° (immediate) response to an antigen. Fixes complement. Cannot cross the placenta. Antigen receptor on the surface of B cells. Monomer on B cell, pentamer with J chain when secreted. Pentamer enables avid binding to antigen while humoral response evolves.

Question 4

IgD

Answer 4

Unclear function. Found on surface of many B cells and in serum

Question 5

IgE

Answer 5

Binds mast cells and basophils; cross-links when exposed to allergen, mediating immediate (type I) hypersensitivity through release of inflammatory mediators such as histamine. Contributes to immunity to parasites by activating eosinophils. Lowest concentration in serum

Question 6

Classic Complement Pathway

Answer 6

IgG or IgM mediated

Question 7

Alternative Complement Pathway

Answer 7

microbe surface molecules

Question 8

Lectin Complement Pathway

Answer 8

mannose or other sugars on microbe surface

Question 9

C3b functions

Answer 9

opsonization

Question 10

C3a, C4a, C5a functions

Answer 10

anaphylaxis

Question 11

C5a functions

Answer 11

neutrophil chemotaxis

Question 12

C5b-9 functions

Answer 12

cytolysis by MAC

Question 13

Opsonins functions

Answer 13

C3b and IgG are the two 1° opsonins in bacterial defense; enhance phagocytosis. C3b also helps clear immune complexes.

Question 14

Inhibitors functions

Answer 14

decay-accelerating factor (DAF, aka CD55) and C1 esterase inhibitor help prevent complement activation on self cells (eg, RBCs).

Question 15

Early complement deficiencies (C1-C4)

Answer 15

Increased risk of severe, recurrent pyogenic sinus and respiratory tract infections. Increased risk of SLE

Question 16

Terminal complement deficiencies (C5–C9)

Answer 16

Increased susceptibility to recurrent Neisseria bacteremia

Question 17

C1 esterase inhibitor deficiency

Answer 17

Causes hereditary angioedema due to unregulated activation of kallikrein ->
bradykinin.

Characterized by decreasedC4 levels.

ACE inhibitors are contraindicated.

Question 18

Paroxysmal nocturnal hemoglobinuria

Answer 18

A defect in the PIGA gene preventing the formation of anchors for complement inhibitors, such as decay-acclerating factor (DAF/CD55) and membrane inhibitor of reactive lysis (MIRL/CD59). Causes complement-mediated lysis of RBCs

Question 19

IL-1

Answer 19

SECRETED BY MACROPHAGES

Causes fever, acute inflammation. Activates endothelium to express adhesion molecules. Induces chemokine secretion to recruit WBCs. Also known as osteoclast-activating factor.

Question 20

IL-6

Answer 20

SECRETED BY MACROPHAGES

Causes fever and stimulates production of acutephase proteins

Question 21

IL-8

Answer 21

SECRETED BY MACROPHAGES

Major chemotactic factor for neutrophils

Question 22

IL-12

Answer 22

SECRETED BY MACROPHAGES

Induces differentiation of T cells into Th1 cells. Activates NK cells

Question 23

Tumor necrosis factor-alpha

Answer 23

SECRETED BY MACROPHAGES

Activates endothelium. Causes WBC recruitment, vascular leak

Question 24

IL-2

Answer 24

SECRETED BY ALL T CELLS

Stimulates growth of helper, cytotoxic, and regulatory T cells, and NK cells

Question 25

IL-3

Answer 25

SECRETED BY ALL T CELLS

Supports growth and differentiation of bone marrow stem cells. Functions like GM-CSF

Question 26

INF-gamma

Answer 26

FROM Th1 CELLS

Secreted by NK cells and T cells in response to antigen or IL-12 from macrophages; stimulates macrophages to kill phagocytosed pathogens. Inhibits differentiation of Th2 cells

Question 27

IL-4

Answer 27

FROM Th2 CELLS

Induces differentiation of T cells into Th (helper) 2 cells. Promotes growth of B cells. Enhances class switching to IgE and IgG

Question 28

IL-5

Answer 28

FROM Th2 CELLS

Promotes growth and differentiation of B cells. Enhances class switching to IgA. Stimulates growth and differentiation of eosinophils.

Question 29

IL-10

Answer 29

FROM Th2 CELLS

Attenuates inflammatory response. Decreases expression of MHC class II and Th1 cytokines. Inhibits activated macrophages and dendritic cells. Also secreted by regulatory T cells

Question 30

Interferon-α and -β

Answer 30

A part of innate host defense against both RNA and DNA viruses. Interferons are glycoproteins synthesized by virus-infected cells that act on local cells, “priming them” for viral defense by downregulating protein synthesis to resist potential viral replication and upregulating MHC expression to facilitate recognition of infected cells.