Humoral immune response Flashcards
Humoral immune response
Part of the adaptive response
Mediated by B cells (B cells differentiate into plasma cells and produce Ab)
Antibodies are the effectors
Goal is to neutralize and eliminate extracellular antigens
If Ag inside of cell, Ab cannot get to it
B cell response to antigens
B cell recognizes microbe/Ab
Helper T cells, other stimuli facilitate activation of B cells
Proliferation
Differentiation to:
- to antibody secreting plasma cells = antibody secretion
- always happens*
- IgG-expressing B cell = Isotype swtiching
- high-affinity Ig-espressing B cell = memory B cell or Affinity maturation
- Other steps depend if T cell involved or not *
Antigens and other signals can:
- stimulate proliferation and differentiation of B cels
- IgM synthesis and secretion (small amount)
- Enhance B cell ability to interact with Th cells (responsiveness, migration)
Multivalent antigens vs proteins
-induce different responses
B cell response to antigens: protein antigens
Th cells help B cells respond protein antigens
T cells can only recognize peptides derived from proteins when presented on MHC molecule
induce a T-dependent B cell activation
B cell response to antigens: multivalent antigens
Polysaccharides, nucleic acids, lipids
Contain the same structural unit repeated multiple times in tandem
Stimulate antibody production without the involvement of Th cells (T-independent activation of B cells)
Can recognize without any type of presentation
B cell response to antigens: T dependent
Follicular B-cells (B2 cells)
B cell response to antigens: T independent
Marginal zone B cells and B1 cells
Innate like B cells: broad specificity BCRs and TLR
T dependent
Protein antigen onto follicular B cells
Helper T cells attach
Isotype switched, high affinity antibodies; memory B cells, long lived plasma cells
T independent
Polysaccharide antigen onto B-1 cells, marginal zone B cells (multivalent, allows cross linkage of B cell receptors) Other signals (like complement protein, microbial product) Mainly IgM, low-affinity antibodies; short-lived plasma cells
Germinal center formation
A. Independent activation of T and B cells
Interaction at the edge of follicles
T cells need to be primed by DCs first
B cells can recognize Ag on own
B. Initial plasma cell activation
Migration of B and T cells to follicle (B cell zone)
Germinal center formation
Germinal centers
Sites where B cells grow undergo maturation
Ag specific B cell activated and stimulated and starting proliferation will produce germinal centers
Round-ovoid clusters of cells
Divided into light and dark zones
Originates from antigen-specific B cells
Places where you will have proliferation afterAg stimulation
“where the magic happens”
Activation of B cells and migration into germinal center
B cell proliferation
Somatic mutation and affinity maturation; isotype switching
Exit of high-affinity antibody secreting cells and memory B cells
Th cells co-stimulation
In primary response: DC presenting Ag to Tcell
In secondary response: already activated T cells
IL-1 interacts with Th cells
T and B cell interaction
CD40/CD40L (CD154) co-stimulation
CD154 (Th cell) ->
- B cell (CD40) : proliferation; Ig production; Ig class switching
- Dendritic cell (CD40): antigen presentation; cytokine production; cell survival
- Macrophage (CD40): cytokine production; activation
Th cell co-stimulation
Activated helper T cell expresses CD40L, secretes cytokines
B cells are activated by CD40 engagement cytokines
B cell proloferation and differentiation
CD40/CD154L co-stimulation
CD40-CD154 (CD40L) up-regulates expression of IL-4 and IL-4 receptors on B cells
Signals from CD40 synergize with IL-4 and IL-5 receptors
B cell activation, memory cell development and Ig class switching
(IL-4 usually secreted by Th2)
Cytokine secretion by Th2 cells: IL-4
Wont have in T-independent
growth and differentiation of B cells
Increased expression of MHC II
Induces Ig class switching
Cytokine secretion by Th2 cells: IL-5
B cell -> plasma cells
Increased IgM and IgG production
IL-5 and IL-4 increase IgE production
Selectively increase IgA production
Cytokine secretion by Th2 cells: IL-6
Needed for final differentiation of B cell into plasma cells
IL-6 and IL-5 increase IgA production
IL-6 and IL-1 increase IgM production
Cytokine secretion by Th2 cells: IL-13
similar to IL-4
Required for optimal production of IgE
Innate co-stimulation
A. C3d bound to microbes
Simultaneous binding of: C3d-CR2 (CD21)
Microbial antigens-BCR
B. PAMPs
Simultaneous binding of: PAMP-TLR
Microbial antigens-BCR
Plasma cells
Develop from antigen stimulated B cells
Spleen, the medulla of lymph nodes, and the bone marrow
10,000 Ig/second
V active metabolically
At least two population:
- (t ind) short-lived population: 1-2 weeks and produces large amounts of antibodies after antigen exposure
-(tdep) long-lived population: months or years and accumulate in the bone marrow (if exposed a 2nd time, have enough Ab to block it w/o becoming sick)
Memory cells
2 populations:
-Long lives resting cells (IgG BCR). Survival do not depend on antigen contact
(if encounter Ag quickly activate and facilitate secondary response)
-large and dividing memory cells (IgM BCR). Survival depends on antigen contact
Survival in humans up to 60 years
Primary vs secondary Ab response
Enough Ag for first time encounter naive B cell. Becomes activated and proliferates. Predominance of IgM in 1st response
Much easier to activate in second response because of memory cells
The magnitude of second response is greater
More IgG produced second time
Primary Ab response
Lag after immunization: usually 5-10 days
Peak response: smaller
Ab isotype: usually IgM > IgG
Ab affinity: Lower average affinity, more variable
Secondary Ab response
Lag after immunization: usually 1-3 days
Peak response: larger
Ab isotype: Relative increase in IgG and under certain situations in IgA or IgE (heavy chain isotype switching)
Ab affinity: higher average affinity (affinity maturation)
B cell receptors
200,000 to 500,000 BCRs in each cell
Antigen binding and signaling components
Immunoglobulin superfamily
Immunoglobulin structure
4 polypeptide chains
- two identical heavy chains
- two identical light chains
- constant and variable regions in each chain
Fab- fragment antigen binding
Fc- fragment crystallizable
Reduce and acidify: isolated chains
Papain: papain fragments (Fab, Fc)
Pepsin: Pepsin fragments (F(ab’)2, pFc’)
Hinge regions: allows arms to move around and adjust
Heavy chains
400-500 amino acids divided in 4-5 domains
1 variable domain (Vh)
3-4 contant domains (Ch)
five types of HC:
-a, y, d, e, u
-differences in aa sequence, domain structure and biological activity
-determines the Ig class
Light chains
220 amino acids in 2 domains 1 variable domain (Vl) 1 constant domain (Cl) Two types of LC: -k, l -functionally identical (can change light chain without changing any function overall
Antigen binding site
Composed by the variable domains in light and heavy chains
(is the cleft between the chains)
-framework regions: hold complimentary regions together kind of and offer stability, allows molecules to remain in same general structure
-Complementary determining regions (hypervariable): may mutations- vary a lot in structure and conformation
CDRs
Sequence variation in 6 to 10 amino acids in three regions of the variable domain Constant regions (framework regions) in between CDR
Will change how attach and fit with epitope : lock and key
BCR’s signal transducing component
The BCR itself cannot signal directly to the cell
Associates with glycoprotein heterodimers formed by pairing CD79a (Ig-a) with CD79b (Ig-B)
Antibodies
Soluble BCRs Divided in 5 classes (type of HC) -IgG- y HC -IgM- u HC -IgA- a HC -IgE- e HC -IgD- d HC
IgG
Y heavy chain- 4 domains 180 kDa MW Very mobile (hinge region) Small size = can move to tissues The most abundant in serum
IgG functions
Most versatile
Neutralization of microbes nad toxins
Opsonization of antigens
Activation of the classical pathway of complement
Antibody dependent cellular cytotoxicity (NK cells) (ADCC)
Neonatal immunity
IgM
u Heavy chain-5 domains in each subunit
5-6 subunits per molecule
900 kDa MW (polymeric form)
No hinge region
IgM functions
Second highest concentration in serum
Major Ig of the primary immune response
More efficient than IgG at complement activation
Rarely enter tissue fluids (bc of size)
IgA
a heavy chain- 4 domains in each subunit
2-3 subunits per molecule
Usually secreted as a dimer (360 kDa)
Produced in body surfaces and transported through epithelial cells into external secretions
IgA functions
Does not activate the classical complement pathway
Mucosal immunity
Neutralization of microbes and toxins
Located in different part of the body too
Secreted by mucosal plasma cells
Binds to receptors (pIgR) on the interior intestinal enterocytes
Passed in vesicles to the cell surface
In the intestinal lumen, the pIgR is cleaves
Secretory component
IgE
e Heavy chain- 5 domains
presents a hinge region
190 kDa molecular weight
Low concentrations in serum
IgE functions
IgE triggers the release of inflammatory molecules from the mast cells
Mediates type I hypersensitivity reactions (allergic rxns)
Responsible in part for immunity to parasitic worms
Shortest half-life of all immunoglobulins (2-3 days)
IgD
d Heavy chain- 4 domains Presents a hinge region 170 kDa MW IgD is mainly found attached to B cells Bonds between HC not strong so degrades easily
IgD functions
Found in horses, cattle, sheep, pigs, dogs, rodents, and primates
Not detected in rabbits or cats
Does not have a clear role
In humans, IgD binds to basophils -> IL-4, IL-1, cathelicidins, and B cell activator factor
Isotype distribution within the organism: IgA
Neutralize things that invade body so it is in secretions and mucosal tissues
Respiratory tract, gut mucosa, mouth etc
Isotype distribution within the organism: IgE
Epithelial, subQ, and some mucosal tissues
Mast cells
Isotype distribution within the organism: IgG
Smallest and can diffuse through tissues- most versatile so it like everywhere
Isotype distribution within the organism: IgM and IgG
in blood
IgM is too big to leave easily
Immunoglobulin varients
Isotype
Allotype
Idiotype
Isotypes
Classes and subclasses (shared via species)- more than one type of IgA or IgG etc
Result of gene duplication
Allotypes
Variations in immunoglobulin amino acid sequences (not associated with Ag specificity)
Ig of one individual may differ from those of another individual of the same species
(minor differences)
Idiotypes
Variations in the amino acid sequences within the variable domains on light and heavy chains
(like for all the different illnesses they have come accross)
Each individual with have a large number of different idiotypes
Class switching
Switch from IgM production to IgA production by deleting intervening heavy chain genes and joining the V genes to the appropriate heavy chain gene
Change the isotype without altering its Ag specificity
Dont change variable domain- bc dont want to change affinity for the Ag- kind of rearrange stuff
Antigen-antibody binding
Interaction between chemical groups on the surface of the antigen and on the CDRs
Non-covalent bonds (need enough to generate a stable binding bc they are weak bonds)
Requires a close conformational fit (lock and key)
Affinity
Refers to the strength of binding of one Fab of an antibody to one antigenic epitope
The sum of attractive and repulsive forces between the epitope antigen-binding
Avidity
Refers to the strength of overall binding of the two molecules
Increasing affinity can increase avidity
Affinity maturation
Antigen concentration declines after the start of the Ab response
-only successively higher affinity cells will bind sufficient antigen to maintain proliferation ( need to be encountering Ag)
High rate of mutation in the germinal centers
-mutants with an advantageous higher affinity will bind well with antigen -> positive selection for persistent clonal expansion
B cell activation by protein antigen and helper T cells
B cells with somatically mutated Ig V genes and Igs with varying affinities for antigen
B cells with high affinity membrane Ig bind antigen on follicular dendritic cells (FDCs) and present antigen to helper T cells
B cells that recognize antigen on FDCs or interact with helper T cells are selected to survive; other B cells die
Antibodies as effectors
Neutralization of microbes and toxins
Opsonization and phagocytosis of microbes
Antibody dependent cellular cytotoxicity
Lysis of microbes
Phagocytosis of microbes opsonized with complement fragments
Inflammation
Complement activation
Antibodies as effectors IgG
Neutralization of microbes and toxins
Opsonization of antigens for phagocytosis by macrophages and neutrophils
Activation of the classical pathway of complement
Antibody-dependent cellular cytotoxicity mediated by NK cells
Neonatal immunity: transfer of maternal antibody across placenta and gut
Feedback inhibition of B cell activation
Antibodies as effectors IgM
Activation of classical pathway of complement
Antibodies as effectors IgA
Mucosal immunity: secretion of IgA into lumens of GI and respiratory tracts, neutralization of microbes and toxins
Antibodies as effectors IgE
Eosinophil and mast cell medicated defense against helminths
Neutralization of microbes and toxins
A. Without antibody: microbe entry through epithelial barrier
With antibody: antibody blocks penetration of microbe through epithelial barrier
B. without antibody: infection of cell by microbes
With antibody: antibody blocks binding of microbe and infection of cells
C. Without antibody: Pathologic effect of toxin
With antibody: antibody blocks binding of toxin to cellular receptor
Receptors for antibodies
Fcy means that its a receptor for IgG, Fce means IgE.
FcyRI has a high affinity for Ig
Receptors for antibodies: FcyRI
High affinity for Ig.
Cell distribution: macrophages, neutrophils, eosinophils
Function: phagocytosis; activation of phagocytes
Receptors for antibodies: FcyRIIA
Low affinity for Ig
Cell distribution: Macrophages, neutrophils; eosinophils, platelets
Function: phagocytosis; cell activation (inefficient)
Receptors for antibodies: FcyRIIB
Low affinity for Ig
Cell distribution: B lymphocytes, DCs, mast cells, neutrophils, macrophages
Function: Feedback inhibition of B cells, attenuation of inflammation
Receptors for antibodies: FcyRIIIA
low affinity for Ig
Cell distribution: Nk cells
Function: Antibody dependent cellular cytotoxicity
Receptors for antibodies: FceRI
high affinity for Ig; binds monomeric IgE
Cell distribution: Mast cells, basophils, eosinophils
Function: activation (degranulation) of mast cells and basophils
Antibody mediated opsonization and phagocytosis of microbes
Phagocyte cell with Fcy recognizes IgG antibody. signals to phagocitize while thing- opsonization
Opsonization of microbe by IgG Binding of opsonized microbes to phagocyte Fc receptors (FcyRI) Fc receptor signals activate phagocyte Phagocytosis of microbe Killing of ingested microbe
Antigen-dependent cellular cytotoxicity (ADCC)
Antibody-coated cell with surface antigen
The surface antigen recognizes IgG
Receptor recognizing Fc portion of Ab attached to Ag.
Nk cell to Fc portion of IgG
leads to killing of antibody coated cell
IgE
and eosinophil-mediated killing of helminths
Th2 cell releases IL-5 which acts on eosinophil
FceRI on the eosinophil recognize FC regions of IgE
binds helminth
Eosinophil activation
Helminth death
