Humoral Immunity Flashcards Preview

Infection & Immunity Block > Humoral Immunity > Flashcards

Flashcards in Humoral Immunity Deck (33):

Humoral Immunity

B lymphocyte-mediated immunity
Immunity that can be transferred from an immune to a non-immune person by transfer of serum that contains antibody; Intravenous immunoglobulin (IVIG)
The gamma-globulin protein fraction of serum contains the antibody activity
Antibody activity = specific recognition and binding to the inducing antigen


Antibody Function

Ab: ability to recognize AND bind the foreign Ag
Highly specific binding site for foreign materials that promotes Ab protective effects such as inhibition of microbial adherence, phagocytosis, etc.


Humoral Immunity and Extracellular Microbes

Humoral immunity protects against extracellular microbes or immunogenic molecules such as bacterial toxins. This includes extracellular phases of viral infection

Ab are in extracellular spaces only and do not cross the membranes
B cells have surface receptors that interact with extracellular substances to cluster the receptors and induces proliferation of those B cells with those receptors on them
Ab will accumulate and concentration will rise in circulation and prevents reinfection of the same pathogens (provides immediate protection once re-exposed)


B cell Maturation

B cells mature in bone marrow
Each clone expresses a unique antigen receptor (surface antibody or immunoglobulin) generated by random DNA recombination of multiple gene segments encoding immunoglobulin protein chains.
~1011 possible different receptor combinations
Immature B cells with autoreactive receptors are deleted by apoptosis (clonal deletion)

Antigen-activated B cells synthesize and secrete a soluble form of this receptor (plasma antibody) which accumulates in fluids and specifically binds and eliminates the stimulating antigen.


Lab Markers for B cells

surface Ig


Clonal Expansion of B cells

B cell with naïve Ig, then see any type of chemical determinant (free or bound unlike T cells that can only see bound) and will undergo initial stages of activation, but to undergo full process must see cytokine and surface to surface from helper T cells (full maturation and proliferation)

Under the influence of cytokines from T cells, they will differentiate along different paths aka T cell dependent maturation

Ag driven: Ab bind and help eliminate Ag, but once Ag drops the stimulus the cells die via apoptosis, but the other B cells will persist as memory cells in case of re-exposure for immediate protection


B cell Affinity

As they are replicating in germinal center, there is a somatic mutational process that mutates the AA sequences in Ag binding site (high rate) to increase Ab for target
B cells can lose affinity altogether, but most of them increase the affinity and continue to bind with highest strength and affinity they continue to proliferate


Ig Class Switching

Ig class switching under T cell control, and regulated in regards to T cell signals to change which Ig’s are expressed


Polyclonal Ab Response

Great diversity of B cells with individual specificities
Large molecules (10000MW), chemically complex, and biologically degradable = foreign molecules; most are large proteins
Ag determinant – only 6 AA in size, so if have protein with 100s of AA long, any of the chemical determinants are multiple stimulators of multiple B cells to cause proliferation and get Ab to each stimulus
Anti-polio for example, do not make just one Ab, you make thousands of different Ab that goes with each different determinant on that virus
Ab has two identical binding sites, but have heterogenous response (many different Ab attack)


Structure of Immunoglobulins

Structural domains have conserved (C) or variable (V) structure between Ig molecules with different antigen specificity
Monomeric Ig has 4 covalently linked polypeptide chains: 2 identical heavy chains (H) and two identical light chains (L)

The sequence is conserved, and at N terminal regions they have variable AA sequence (compare between two Ab, this is where you see the structural differentiation between the two)

Fab Region: for Ab binding

Tail = class and represent Ab differences = Fc region = constant fragment; this region is C1 binds of the classical complement pathway; Fc regions cluster together and C1 binds to Fc region to become activated; also involved in opsonization because phagocytes have an Fc receptors that recognize the tail of the Ig, and phagocytes will bind and forms a bridge between bound substance and promotes phagocytosis


Antigen Binding Sites

VH and VL domains fold to form antigen binding sites that resemble pockets, grooves, open faces, or extrusions. Each monomeric Ig has 2 identical binding sites.

High specificity of Ab and how they work
Bind small chemical determinants, but very specific = lock and key 3D complimentary interaction
Reaction is non-covalent


Ig Class

Ig class (isotype) is based on similarities of H chain structure.
Secreted forms of IgA and IgM exist as multimers.

Most Ig are monomeric, but IgA is a dimer (J chain protein that hooks them together) and IgM is a pentamer and so large it is confined to intravascular spaces

Initially all B cells will secrete is IgM, but then class is based on conserved regions on heavy chains and encoded on different genes than the variable regions that determine Ag specificity, so under T cell influence, the B cell can switch IgM to any of the other Ig, and conserves the specificity of the conserved part of the protein



IgA: monomer, dimer; mucosal immunity



IgD: naïve B cell antigen receptor



IgE: monomer; mast cell activation (type I hypersensitivity), defense against helminthic parasites



IgG: monomer; opsonization, complement activation, Ab dependent cell mediated cytotoxicity, neonatal immunity, feedback inhibition of B cells; long term immunity with longest concentration and half life



IgM: pentamer; naïve B cell antigen receptor and complement activation

IgM is expressed with a transmembrane domain not secreted, but following activation it ceases to express the transmembrane domain and it becomes the secreted form of protein
B cells have a period where they can have both surface Ig and are secreting at the same time, but once fully mature to plasma cell can only secrete
IgM is a monomer when attached to B cell, but once secreted is a pentamer


Plasma Half Life and Concentration of Ig

Plasma Half Life (days): IgG > IgA > IgM > IgE

Plasma Concentration: IgG > IgA > IgM > IgE > IgD


Effector Functions of Ab

Plasma cells secrete them and stay in plasma
Ab specifically recognize and bind inducing Ag/immunogen

Toxic molecule: Ab binding mediates toxicity, and blocks the toxic activity aka neutralize
Ab binding and coating to viral/bacterial surface can block microbial attachment to tissues

Opsonization: Ab promote phagocytosis via the Fc Ig region by coating the pathogen
Any cell with Fc receptors, especially NKC, which are not phagocytic or specific but can be targeted to specific cell by an Ab

If viral infection in cell, if Ab binds to surface similar to opsonization, NK cells will recognize the Ig through Fc receptors and bind that target and mediate killing = Ab mediated killing
Once you make a specific Ab, the IgG/IgM can activate the classical pathway of complement via C1q of the pathway
All important in extracellular spaces against extracellular microorganisms


Ag Presentation by B cells to CD4 T cells

Antigen binding causes B cells to express MHC II-peptide antigen complexes, B7, CD40, and cytokine receptors on their cell membrane for receipt of T helper co-stimulatory signals (CD40L and cytokines).

Ag uptake, because B cells are not phagocytic, but have a receptor that binds to foreign material and can undergo receptor mediated endocytosis and the B cell expresses MHC II once proteins are processed and then receive help from the CD4 T cell; costimulatory exchanges involving B cell CD28 and CD40, and then CD40 ligand on the T cell and finally release of GFs from T cell that induces B cell growth and differentiation


CD4 T cell Activation of B cells

Antigen-specific CD4+ T helper lymphocytes provide help for B cell activation.
Th cells recognize peptide-MHC II presented by B cells and provide signals for B cell proliferation, isotype switching, and differentiation into plasma cells and memory cells
Required B7-CD28 interaction for Th activation


T cell Dependent B cell Activation

Naive B cells use surface IgM (and IgD) to bind and internalize specific antigen for processing and presentation of foreign peptide determinants with MHC II to TH cells (T helper lymphocytes)

B cell antibody production to most complex protein immunogens is dependent on:
1st: Specific antigen recognition and crosslinking of surface immunoglobulin (B cell receptor complex)
2nd: T helper cell-derived co-stimulatory signals via B cell-T cell contact (CD40L-CD40 ligation, cytokines, etc.)


Lymphatic Tissue Interaction of T and B cells

T cell is being independently activated by seeing peptides on dendritic cell in T cell rich areas
B cells are seeing Ag and processing peptide in follicle
Early activated T and B cells will migrate towards each other and reciprocally upregulate chemokine receptors and get early Ab production and some B cells migrate back into follicle and begin massive proliferation to form the germinal center and some migrating follicular T helper cells maintain this processes by providing ongoing help to continue proliferation and affinity maturation


Ag Exposure in B and T cells

B cell is repeatedly being exposed to peptides via follicular dendritic cell in the germinal centers and can trap Ab-Ag complexes to bind on dendritic cell and hold Ag for long periods of time for repeated exposure for B cells and lead to affinity maturation

T cell can sense many different peptides, and must get infection to get immunity, but you have the ability to respond to different immunogens even before exposed, but develop better response the next time


Location and Mechanism of Class Switching

The class switching occurs in the lymph nodes when B cells proliferate and mature
It is a gradual process, and can even happen in periphery
Constant region genes are lined up and must switch in order, and cuts out the preceding class to make the next class, and once cut out cannot switch back to that class
B cells cannot secrete Ig’s until mature


T-Independent Immunogens

Polymeric antigens (polysaccharides, nucleic acids) and B cell mitogens can directly stimulate B cell activation, but without T help little or no Ig class switching, affinity maturation, or B cell memory responses are generated. (mostly IgM class responses).
Pure polysaccharide vaccines are poorly effective because little IgG is stimulated by the T-independent immunogen
Protein-polysaccharide conjugate vaccines provide T and B cell epitopes for good IgG and memory responses

C3D – degradation product, and B cells have CD21 which is a receptor for CD3, so if B cell simultaneously binds microbe and C3D, it physically cross links the molecules and causes clustering and signaling cascades for Ab production
Some examples for microbial patterns, if Ab binds target and PAMPs bind TLRs these two signals can drive T cell independent Ab production


T cell Dependent vs. Independent B cell Activation

T cell dependent are best long term immune response because mediates so many things
IgG is the highest in concentration, more important functions, etc. and to be able to switch is from T cell signal from protein stimulus (protein Ag)
Many of the important Ag on the surfaces are carbohydrates, and since T cells ONLY respond to proteins, it cannot respond to these
Without T cell signal, B cells cannot get activated and will only make IgM
Polysaccharide vaccines with covalent protein bound and peptide determinant – provide T and B cell determinant and Ig class switching
Can make Ab responses to non-proteins called T independent responses, but little affinity maturation, class switching, or B cell memory cell production occurring


Ig Heavy Class T cell Dependent Switching

The Ig heavy chain class switching is driven by T cells and cytokines
B cell can change which heavy chain it expresses and all depends on which cytokines it being secreted by the T cell
There is no changing of the light chain type or class
Why would you switch the conservative area but not the Ag specific area? To make protective response to a specific challenge; want to conserve specificity binding site
Order of Switching: IgM, IgG, IgE, IgA


Antibody Memory

Antibody secretion is sustained by plasma cells in medullary cords of lymph nodes (2-4 day lifetime) and in bone marrow (months to years after initial response)
Possible replenishment by memory cell differentiation

B memory cells recirculate through B cell zones of secondary lymphoid tissues
100X increase in frequency of antigen-specific B cells after primary response

Follicular dendritic cells (FDC) may serve as a reservoir of antigen (trapped as immune complexes bound to FDC membrane FcR and CR) for long term re-stimulation of memory cells

T and B cell memory are maintained for life


Primary vs. Secondary Response

First time exposed to Ag: primary response and then in future is secondary
Timing and type of Ab is different
Lag: exposure to development of Ab levels – 1-2 weeks/5-10 days
Secondary response: 1-3 days in circulating Ab levels
Peak level is bigger in secondary
IgM is first, and the longer the response occurs then switching occurs
Repeated exposure: go to predominate class
Affinity maturation: Ab produced early have low affinity, but Ab accumulates during another exposure, the higher the affinity
Prime the immune response with immunizations


Diagnostic Application of Antigen-Antibody Reactions

Purified antigen-specific antibodies are used for very sensitive and specific lab detection and quantitation of
Infectious agents
Drugs, hormones, toxins, plasma proteins
Tumor antigens
Tissue Transplantation antigens, blood groups

Purified antigens can be used to detect specific antibody responses in plasma as evidence of present or past immune responses


Cross Reactivity

One antibody can bind to 2 different immunogens if they share a common epitope

Patient serum (polyclonal mixture of antibodies) may show a false-positive test for specific antibody activity to a microbe because of cross-reactivity of an antibody to a shared immunogen between different microbes.

Make heterogeous response and making many Ab; may have two viruses that share some structures, but also have unique proteins; if you make an immune response to one, you will also make Ab that can be used for both viruses and get cross reactions and won’t be able to detect specific virus alone
Influenza: has many types, lots of shared molecules between organisms


Diagnostic Significance of Antibody “Titers”

“Titer”= reciprocal of highest dilution of serum that gives a physical reaction with specific antigen in a laboratory test

In an immunocompetent person, antigen X exposure causes an increase in specific antibody “titer” to X (anti-X) over the course of an illness

Elevated antigen-specific plasma IgM levels (titers) indicate recent antigen exposure; short half life so indicates recent exposure if present

Elevated antigen-specific plasma IgG levels (titers) indicate current or past antigen exposure; long half life and indicates memory Ab and from long time ago exposure