Humoral Immune Response

Question 1

Describe antibodies.

Answer 1

-antibody mediated elimination of antigens:
>antibodies only defense mechanism to combat microbes in lumen of mucosal organs & in fetus/newborn

Question 2

Describe how antibodies neutralize microbes & microbial toxins.

Answer 2

Antibodies against microbes/microbial toxins block the binding of microbes/toxins to cell receptors

Question 3

Describe leukocyte Fc receptors + NK (ADCC).

Answer 3

-leukocyte Fc receptor: FcyRI (CD64) receptor (IgG1 & IgG3) = macrophages & neutrophils +
NK
Fcδ (IgE) - eosinophils

Question 4

Describe complement activation - classical pathway.

Answer 4

Question 5

Describe the different types of antibodies & their mechanisms.

Answer 5

*exogenous antigen destroyed by antibodies made by B cells against:
1) bacteria
-neutralization of toxins/enzymes
-killing = complement (classical pathway)
-ADCC = FCy receptor (IgG) & Fcδ (IgE)
2) virus
-virion = complement or phagocytosis
3) parasites
-Th2 = IgE + eosinophils
4) tumors
-not in solid cancers
-lymphosarcomas = complement

Question 6

Describe the BCR.

Answer 6

-200-500k BCRs (soluble)
-4 peptide chains (2 heavy & light chains)
>Iga (CD79a)
>IgB (CD79b)

Question 7

Describe costim of B cells.

Answer 7

-helper T cells = presented w antigen by APC
-B cell = APC & receive costim from same T cell

Question 8

Describe B cells activating Th cells.

Answer 8

-B cells can activate Th cells w 1/1000 of antigen conc required to activate macrophages
-Th2 cytokines (IL4, IL5, IL6, IL13)

Question 9

Describe B cell activation & antibody production.

Answer 9

-activation of B cells -> prolif & differentiation into antibody secreting plasma & memory cells
-humoral immune resp initiated by specific B cell recog of antigen in secondary lymphoid organ
-antigen bind to IgM & IgD on mature, naive B cells = signal required for prolif & differentiation into plasma cells
-antibody secreted by plasma cell = same specificity as OG Ab that was the antigen receptor on surface of naive B cell
-single B cell in a week makes 5000 antibody secreting cells (each secrete 2000 Ab per sec)

Question 10

Describe T dependent antibody responses.

Answer 10

-response to protein antigens where T cells help B lymphocytes to make antibodies
-activated B cells:
>make antibodies other than IgM (IgG, IgA, IgE) = ‘heavy chain isotope (class) switching’
>antibodies bind w high affinity = ‘affinity maturation’
>long lived plasma cells = ‘memory B cells’

Question 11

Describe T independent antibody responses.

Answer 11

-multivalent antigens w repeating determinants (ex. Polysaccharides) that activate B cell without T cell help
-rapid
-low affinity IgM antibodies

Question 12

Describe primary & secondary antibody responses to antigens.

Answer 12

1) primary = activation of unstim naive B cell
2) secondary = stim of expanded clones of memory B cells

Question 13

Describe the different types of B cells found in T independent VS T dependent cells.

Answer 13

1. T dependent
   -follicular B cells in secondary (peripheral) lymphoid organs make antibody resp to antigen w help from helper T cells
2. T independent
   -marginal zone B cells in spleen (+ other lymphoid tissue), B1 cells in mucosal tissue, & peritoneum recog multivalent antigens like blood borne polysaccharides

Question 14

Describe antigen capture & delivery to B cells in the lymph node.

Answer 14

*antigens that do antibody resp vary in size & comp = free/bound to antibodies
-pathways of antigen delivery:
1. Antigen from tissue to LN by afferent lymphatic cells drain into subcapsular sinus & soluble antigens reach B cell zone
2. Subcapsular sinus macrophages capture lg microbes & antigen-antibody complex & go to follicles
3. Antigens in immune complex = enter spleen bind to complement receptors (CR2) on marginal zone B cells & transfer to follicular B cells
4. Polysaccharide antigens = captured by macrophages in marginal zone of splenic lymphoid follicles & transferred to B cells
antigen presented intact & not processed by APC

Question 15

Describe BCR.

Answer 15

*BCR = mature B cells + membrane Ig molecules that bind antigens + Iga & IgB proteins that deliver signals for B cell activation
-BCR complex 2 roles in B cell resp:
1. Bind of antigen to receptor = signals to B cells that initiate activation
2. BCR internalizes bound antigen into endosomal vesicles & if Ag is protein its processed into peptides that’s presented by MHC II on B cell surface for recog by helper T cells

Question 16

Describe the B cell activation.

Answer 16

A) B cell activation by CR2 coreceptor on B cells = recog complement frag covalently attached to antigen or part of immune complex w antigen
B) microbial products engage TLRs on B cells = enhance B cell activation

Question 17

Describe the summary of events during T cell dependent antibody responses.

Answer 17

A) immune resp initiated by recog of antigen by B cells & CD4 T cells - activated lymphocytes migrate to each other & interact at T & B cell zones
B) initial T-dependent B cell prolif & differentiation form extrafollicular focus where B cells prolif, undergo isotype switching, & differentiate into plasma cells

Question 18

Describe the sequence of events during T cell dependent antibody responses.

Answer 18

1. protein antigen independently recog by T & B lymphocytes in 2ndary lymphoid organs
2. 2 activated cell types interact to start humoral immune resp
3. naive CD4 T cells activated in T cell zone by antigen presented by DCs
4. activated T helper cells & B cells migrate to each other & interact at follicle where initial resp starts & some migrate into follicles to make germinal centers where specialized antibody resp made

Question 19

Describe antigen presentation on B cells to helper T cells.

Answer 19

-antigens recog by membrane Ig = endocytosed & processed = peptide frag presented w MHC II
-helper T cells recog MHC-peptide complex on B cells & stimulate B cell resp

Question 20

Describe the role of CD40L:CD40 interaction in T dependent B cell activation.

Answer 20

-upon activation, helper T cells express CD40L & CD40 receptor on antigen stimulated B cells = induce B cell prolif & differentiation in extrafollicular foci & then in germinal centers
-CD40 = member of TNF

Question 21

Describe extrafollicular B cell activation.

Answer 21

-B cell activation in extrafollicular focus = early antibody resp to antigens & sets up germinal center reaction
-extrafollicular foci of T dependent B cell activation = low affinity Ab (limit spread of infection by circulation)
-focus makes 100-200 antibody secreting plasma cells
-spleen = extrafollicular foci develop in outer portion of T cell rich PALS or between T cell zone & red pulp (PALS foci)
-T dependent foci in medullary cords of LN

Question 22

Describe T follicular helper cells.

Answer 22

-4-7d after antigen exposure = activated antigen specific B cells outside follicle induce previously activated T cells to differentiate into T cells that express CXCR5 (chemokine) = germinal center formation
-differentiation of T cells from naive CD4 T cells requires:
1. Initial activation by antigen presenting DCs
2. Activation by B cells
*can be Th1, Th2, Th17 depending on strength of initial interaction between peptide MHC II complex on DCs & TCR on naive CD 4 T cell

Question 23

Describe the germinal center reaction.

Answer 23

-helper T cell dependent antibody resp:
>affinity maturation, long lived plasma & memory B cells, isotype switching = occur in germinal centers created within lymphoid follicles
-B cell differentiation & selection of cells w high affinity antigen receptors occur in germinal center reaction

Question 24

Describe the germinal center reaction steps.

Answer 24

1. Initiation of germinal center by T cells
2. Entry of B cells into GC
3. B cell prolif
4. Somatic mutation in Ig genes
5. B cell migration in GC
6. Selection of high affinity B cells
7. Repetitive mutation & selection
8. Differentiation into long lived plasma cells
9. Memory B cell formation

Question 25

Describe heavy chain isotype class switching.

Answer 25

-T dependent resp
-activated IgM & IgD B cells
-make Ab w heavy chains of diff classes like: y, a, ε
-B cells change isotype of Ab production by changing constant regions of heavy chains but specificity of Ab is same

Question 26

Describe the different types of Ig.

Answer 26

*isotype switching in resp to diff types of pathogens regulated by cytokines made by T helper cells activated by those pathogens
-switch from IgM to IgD = T dependent antibody resp against bacteria & virus
1. IgG = phagocytosis of opsonized microbes, activate complement, transferred via placenta
2. IgE = humoral resp to helminthic parasites (elimination & hypersensitivity reaction)
3. IgA = B cells in mucosal tissue - transported thru epi into mucosal secretion to prevent microbes from entering epi
4. IgM = complement activation

Question 27

Describe CD40 signals.

Answer 27

-work w cytokines to induce isotype switching
>’switch recombination’ = lg heavy chain DNA in B cells cut & recombined
>AID (activation induced deaminase) key enzyme
—expression induced in activated B cells by CD40 signals from Tfh cells

Question 28

Describe affinity maturation: somatic mutation of Ig genes & selection of high affinity B cells.

Answer 28

-increased affinity of Ab for antigen as T dependent humoral resp continues
-result of somatic mutation of Ig genes by selective survival of B cells that make Ab w high affinity
-generate Ab w increased ability to bind to antigens & neutralize/elim microbes/toxins

Question 29

Describe B cell differentiation into antibody secreting plasma cells.

Answer 29

-morphologically distinct, terminally differentiated B cells committed to antibody production
-generated after activation of B cells thru signals from BCR, CD40, TLRs & cytokine receptors
2 types:
1. Short lived: made in T independent & early T dependent resp in extrafollicular B cell foci & found in secondary lymphoid organs & peripheral nonlymphoid tissue
2. Long lived: T dependent germinal center resp to antigen
>signal from BCR & IL21 make plasma cells & their precursor ‘plasmablasts’ (earliest cell in antibody secreting cells)

Question 30

Describe plasmablasts.

Answer 30

-made in germinal centers & enter blood where they stop dividing & differentiate into long lived plasma cells
-2-3wk after immunization w T cell dependent antigen = bone marrow major site of Ab production
-plasma cells in bone marrow secrete Ab for decades after Ag no longer present & can provide immediate protection if Ag is seen again
-half Ab in blood is made by long lived plasma cells
-secreted antibodies enter circulation & mucosal secretions but mature plasma cells dont recirculate

Question 31

Describe the change of membrane to secreted form.

Answer 31

-differentiation of B cells into antibody secreting plasma cells:
>structural alteration in ER
>secretory pathway
>change in Ig heavy chain from membrane (B cells) to secreted form (plasma cells)
—change in carboxy terminal of Ig heavy chain protein
—caused by alternative RNA processing of heavy chain mRNA
>increased Ig production
-cell enlarges = ER & golgi complex prominent

Question 32

Describe the generation of memory B cells.

Answer 32

-during germinal center reaction
-rapid resp to subsequent intro of Ag
-made during T dependent immune resp
-some IgM expressing memory B cells made without T cell help & no somatic hyper mutation (T independent)
-memory cell live long & without continuous Ag stim bc of high levels of BCL2 (anti apoptotic protein)

Question 33

Describe T independent antigens.

Answer 33

-non protein antigens: polysaccharide, lipids, nucleic acids
>stim antibody production in the absence of helper T cells
>’thymus independent’
-antibodies produced in absence of T cell help are low affinity IgM & limited isotype switching to IgG/IgA

Question 34

Describe the subsets of B cells that respond to T independent antigens.

Answer 34

-marginal zone & B1 subsets of B cells imp for antibody resp to TI antigens
-marginal zone B cells = resp to polysaccharide
-after activation = diff into short lived plasma cells that make IgM
-B1 cells = lineage of B cells that resp to TI antigens in peritoneum & mucosal sites

Question 35

Describe the mechanisms of TI antibody resp.

Answer 35

-TI antigens = polysaccharide, glycolipids, nucleic acids NOT MHC MOLECULES = NOT RECOG BY CD4 T
-TI antigens multivalent w repeated epitopes
-polysaccharide = activate complement system by alternative or lectin pathway by gen C3d = binds to antigen & recog by CR2 = B cell activation

Question 36

Describe protection mediated by TI antibodies.

Answer 36

-bacterial cell wall polysaccharide
-humoral immunity against encapsulated bacteria
-TI antigens gen natural antibodies in circ of normal individuals = made without overt exposure to pathogens

Question 37

Describe the antibody feedback regulation of humoral immunity resp by Fc receptors.

Answer 37

-secreted antibodies inhibit B cell activation by making antigen-antibody complexes that bind to antigen receptors on antigen specific B cells
-antibody feedback = downreg of antibody production by secreted IgG antibodies
-IgG antibodies inhibit B cell activation by making complexes w the antigen & bind to B receptor for the Fc portion of the IgG = FCy receptor II
>B cell membrane lg & receptor on B cells for Fc portions of IgG called FcyR IB clustered by antibody antigen complex
>activated inhibitory signaling cascade thru cytoplasmic tail of FcyR IB that terminates the activation of B cell