B Lymphocytes & Humoral Immunity (18)

What is the predominate isotype in the serum & lymph?

IgG

Which antibody crosses the placenta and provides passive immunity to newborns?

IgG

During primary response, what is the first secreted antibody by the plasma cells?

IgM

What is the first antibody made/expressed within the bone marrow?

IgM

Mature naive B cells express what 2 antibodies?

IgM & IgD

Which antibody isotypes are produced during isotype switching?

Isotyoe switching occurs at the level of the DNA. - Mainly IgG is produced for systemic immune system & dimeric IgA for mucosal immune system. - Plasma cells and memory cells express the new isotype that has the same antigen specificity as IgM & IgD.

Describe the process of isotype switching.

It is a change from IgM to another isotype. - The antigen specificity remains the SAME 1.) Constant region genes for heavy chain are next to each other and 3' of the variable region genes. 2.) A switch region is in front of each constant region gene, except Cδ for IgD. 3.) The µ switch region pairs with another switch region forming a loop. 4.) The loop is excised (deleted), and intervening DNA is lost. 5.) The gap in the chromosome is ligated (repaired) 6.) the cells can NEVER express IgM or IgD again, because those constant region genes are gone. Thus, isotype switching is PERMANENT.

What does the expression of a variable region require?

It requires the V & J regions of the light chain to be contiguous and the V, D, and J regions of the heavy chain to be contiguous.

What is the main contributor to antibody diversity?

Junctional diversity

What is the valence or valency of antigen receptors?

Valence or valency of antigen receptors, including antibodies, refers to number of antigen-binding sites within one receptor. The number of antigen-binding sites determines the number of antigens that one antibody molecule can bind.

What is somatic hypermutation?

Variable region genes of light and heavy chains undergo point mutations by a process called somatic hypermutation. Some of the mutated immunoglobulins have increased affinity.

What parts of the antibody do NOT influence the isotype?

Variable regions and light chains

Where is the antigen binding site located on an antibody?

Variable regions of the light and heavy chains form the antigen-binding site.

What are the effector functions of pentameric IgM?

complement activation, neutralization (agglutination)

What are the effector functions of IgG?

complement activation, ADCC, opsonization, neutralization, neonatal immunity

Which 2 antibody isotypes are mainly found in the secreted form?

pentameric IgM & dimeric IgA

Gene rearrangements cause antibody diversity in what type of cells?

pre-B cells in the bone marrow

Describe the process of B cell maturation.

- B cells differentiate and mature within the bone marrow. IgM is the first expressed isotype. - Mature B cells exit the bone marrow and are naive, i.e. they have not yet encountered antigen. - The circulatory system delivers B cells to the secondary lymphoid organs and tissues (spleen, lymph nodes, MALT) where they encounter antigen.

Where is IgA distributed in the body?

- Dimeric IgA: part of mucosal secretions of the mucosal immune system. - Monomeric IgA: Serum & lymph

Describe the structure of secreted IgM.

- Plasma cells synthesize a polypeptide called J chain (joining) that holds 5 IgM molecules together forming a pentamer. - Disulfide bonds cross-link the IgM molecules to each other and to J chain. - Secreted IgM has a valence of 10

Which antibody isotypes are present during the primary and secondary responses for T-dependent antigens?

-Primary responses to T-dependent antigens also involve relatively low affinity IgM. - During secondary response, memory B cells respond. ** Plasma cells secrete mainly IgG or IgA due to isotype switching & the effector functions of secreted antibodies change.

What are 3 effector functions of secreted antibodies that are used by innate leukocytes to enhance their functions?

1) Neutralization 2) Opsonization 3) Complement activation

Innate leukocytes borrow antibodies to enhance their opsonization functions. How does this work?

1. Similar to complement, antibodies coating a pathogen promote phagocytosis of pathogens by macrophages and neutrophils and killing of pathogens. 2. In the case of antibodies, phagocytosis is through Fc receptors for IgG and monomeric IgA

Describe the structure of secreted IgA.

1.) Plasma cells in mucosal-associated lymphoid tissues also synthesize J chain. 2.) In this case, J chain holds 2 IgA molecules together forming a dimer with a valence of 4. 3.) Dimeric IgA is part of the mucosal immune system, and is the most abundant antibody in the body.

Describe the basic structure of an antibody.

1.) The "basic unit" of an antibody molecule has 4 polypeptide chains: 2 identical heavy chains & 2 identical light chains. 2.) The light chains are composed of two domains and the heavy chains contain at least four domains. 3.) The four polypeptides are held together by disulfide bonds that join the two heavy chains and each heavy-light chain pair. 4.) The hinge region is where the two heavy chains are held together. The hinge region is flexible allowing several possible configurations of immune complexes (antigen-antibody complexes). 5.) Each chain has a variable region consisting of one domain at the amino terminus. 6.) Each chain has a constant region at the carboxyl terminus

Describe the process of B cell Activation.

1.) The circulatory system delivers mature naive B cells to the secondary lymphoid organs and tissues (spleen, lymph nodes, MALT) where they encounter antigen. 2. ) Mature naïve B cells express both IgM and IgD on the cell surface. - Both isotypes act as B cell antigen receptors, and have the same antigen specificity (same variable regions & same light chains) - Co-expression of IgM & IgD is due to how mRNA is processed. 3.) Antigen binding to B cell receptors induces cell proliferation, and the cells stop expressing IgD. 4.) After cell proliferation, some cells mature into plasma cells that secrete pentameric IgM. In contrast, IgD is rarely secreted. 5.) Plasma cells do NOT express surface immunoglobulins. 6.) Later in the response, the isotypes change called isotype switching.

A variable region gene is composed of gene segments, and a complete variable gene requires one of each segment. What are these segments?

1.) Variable region for light chain consists of a V (variable) gene segment and a J (joining) gene segment. 2.) Variable region for heavy chain consists of V, J, and D (diversity) gene 3) There are multiple V, J, and D gene segments, which tend to be in clusters and the combination of different gene segments increases diversity of the variable region. These genes segments are separated from each other, and expression of a variable region requires that the gene segments be contiguous.

What are the effector functions of IgD?

Activation of naive B cells

What is affinity maturation?

After isotype switching from IgM to IgA and/or IgG, these antibodies have progressively higher affinity for the antigen during the course of the immune response. - B cells undergo more rounds of division, and the antigen selectively activates cells expressing B cell antigen receptors with the highest affinity. ****Affinity maturation is due to somatic hypermutation.

What determines if an antibody will be secreted or membrane bound?

All isotypes have a membrane-bound form and a secreted form. - The 2 forms are determined by the carboxyl terminus of the heavy chain. - Each isotype has separate membreane-coding and secretion-coding segments & which form is made depends on how the mRNA is processed.

Antibody molecules are grouped into 5 isotypes or classes based on what?

Antibody molecules are grouped into 5 isotypes or classes based on the constant region of their heavy chains.

What are T-independent antigens and some characteristics of them?

B cells can respond to some antigens WITHOUT interacting with T helper Cells, and such antigens are called T-independent antigens. - These antigens are usually bacterial molecules with a repetitive structure leading to extensive cross-linking of B cell receptors - These antigens bypass needing signal 2 from CD40 - The resulting antibody is mainly LOW affinity IgM & few memory cells are generated

How do secreted antibodies help enhance the complement activation through classical pathway for innate leukocytes?

Complement activation through classical pathway by IgM & IgG. This leads to lysis and ingestion of the bacteria

What is the predominate isotype in mucosal secretions?

Dimeric IgA

What is the structural form of IgA within saliva as part of the oral immunity?

Dimeric IgA bound to the secretory piece

What is the most abundant antibody in the body?

Dimeric IgA which is part of the mucosal immune system is the most abundant antibody in the body.

When do memory B cells respond?

During the second response

What is combinatorial diversity?

Each antibody molecule has a unique specificity due to the variable regions that form the antigen-binding site. - A variable region from both the heavy and light chains are needed. The 2 chains increase diversity. - Each chain locus has multiple variable region genes located 5' of the constant region genes. -λ and κ light chains have their own set of variable region genes, and either chain can associate with a heavy chain. This also increases diversity.

Describe the constant regions of the heavy and light chains.

Each chain has a constant region at the carboxyl terminus. 1.) The amino acid sequence of the constant region is highly conserved from one chain to another. 2.) The constant regions do NOT influence antigen specificity. 3.) The light chain has 1 constant region domain, whereas heavy chain has at least 3 constant region domains.

What do the variable regions of an antibody do?

Each chain has a variable region consisting of one domain at the amino terminus. 1.) The amino acid sequence of the variable region varies tremendously from one chain to another. 2.) The variable regions determine the antigen specificity of the antibody molecule. 3.) variable regions of the light and heavy chains form the antigen binding site. 4.) The "basic unit" has 2 identical antigen-binding sites, and thus, is bivalent (can bind 2 antigens)

What is junctional diversity?

Each gene segment has a recombination signal sequence (RSS). 2 RSS from different gene segments pair forming a loop, and the intervening DNA is deleted. Repair of the gap is "sloppy," and new nucleotides are introduced at the junction further increasing diversity. The process is random and independent of antigen. The shuffling of the gene segments is the MAIN contributor to antibody diversity and is called junctional diversity.

Isotype switching rarely occurs for which type of antigen?

For T-independent antigens, isotype switching rarely occurs because of the lack of cytokines from helper T cells.

Which antibody is present in breast milk providing passive immunity to infants?

IgA

Why isotype of antibody is very rarely secreted?

IgD

What occurs in the bone marrow precursor B cells during recombination?

In bone marrow precursor B cells, the DNA reorganizes (rearranges) by recombination. - For light chains, any V and virtually and J can recombine -- 1 rearrangement - For heavy chain, any V, any D, and any J can recombine -- 2 rearrangements ** First a D & J recombine, and then a V rearranges to the DJ segment.

Leukocytes of innate immunity express Fc receptors for which isotypes of antibodies?

Leukocytes of innate immunity express Fc receptors for IgG, monomeric IgA, and IgE. 1.) Innate leukocytes use antibodies to enhance their functions. 2.) This situation gives innate leukocytes antigen specificity.

Which antibody class do mast cells and basophils express Fc receptors for?

Mast cells and basophils express Fc receptors for IgE, and a vast majority IgE in the body is bound to these cells. - Antigen binding cell-associated IgE induces degranulation releasing inflammatory mediators from these cells. - Histamine and other mediators cause an allergic reaction, in particular Type I or immediate hypersensitivity.

How do NK cells help with antibody-dependent cell mediated cytotoxicity?

NK cells express Fc receptors for IgG, and bind and kill infected cells coated with antibodies.

Under normal conditions should there be any antibodies in the CNS?

NO!

Which part of the antibody is involved in isotype switching?

Only the constant region of heavy chain is involved. Light chains and variable region of heavy chain remain the same.

Do plasma cells express membrane-bound or secreted antibodies?

Plasma cells do NOT express surface (membrane-bound) immunoglobulins. They express secreted ones.

For T-dependent antigens, what acts as the primary and secondary signal?

Primary signal: B cell receptor is cross-linked by a T-dependent antigen Secondary signal: Helper T cells provide the costimulatory signal (signal 2) to B cells binding CD40 on B cells. --Helper T cells also secrete cytokines that promote B cell proliferation, isotype change, and memory cell development.

What is the valence of secreted vs. membrane bound IgM?

Secreted IgM has a valence of 10 Membrane bound IgM is a bivalent monomer (valence of 2)

What is the function of secreted antibodies?

Secreted immunoglobulins have effector functions mainly mediated through the Fc portion of constant region of heavy chain. Effector functions vary among isotypes

What helps the immune system keep up with mutating pathogens?

Secreted immunoglobulins tag pathogens for destruction or elimination by complement-mediated cytolysis or by leukocytes of innate immunity. As the immune response proceeds, antibodies of progressively higher affinity for the pathogen are produced helping the immune system to keep up with mutating pathogens.

Where is IgM distributed in the body?

Serum

Where is IgG found distributed in the body?

Serum, lymph, tissues & crosses placenta

What causes affinity maturation?

Somatic hypermutation

Where is IgE distributed in the body?

Submucosal tissues & skin

What is the function of membrane-bound antibodies?

Surface immunoglobulins when antigen is bound provide the primary signal for B cell activation.

What is the B cell antigen receptor?

The B cell antigen receptor is the membrane-bound form of antibody also called surface immunoglobulin.

Where is the Fab portion of an antibody located and what does it do?

The Fab portion (Fragment Antigen Binding) consists of a light chain, and the variable region and 1 constant region domain of a heavy chain. It has 1 complete antigen-binding site.

Where is the Fc fragment of the antibody located and what does it do?

The constant region domains of the heavy chains at the carboxyl end form the Fc portion or fragment (Fragment Crystalizable). 1.) These domains do NOT associate with light chain. 2.) The Fc portion confers important effector functions of secreted antibodies.

What are the different kinds of antibodies called?

The different kinds of antibodies are called isotypes or classes. Various isotypes have different major functions.

What are the 5 isotypes or classes of antibody molecules?

The heavy chains are µ, δ, γ, ε and α forming IgM, IgD, IgG, IgE and IgA isotypes of antibody molecules, respectively. (GAMED) - Because the constant regions of the isotypes differ, their effector functions differ.

What is the major function of B cells?

The major function of B cells is to produce antibodies also called immunoglobulins.

What are the 2 different light chains called?

There are two different light chains called λ and κ. - Either light chain can associate with any heavy chain, and, thus, all isotypes can contain either λ or κ light chains

Where are the 2 heavy chains of an antibody held together?

They are held together by disulfide bonds in the hinge region

What is neutralization?

This is used to prevent harmful agents from interacting with host cells. -Its mediated by VARIABLE regions (antigen-binding site) -Antibodies bind bacterial toxins and inactivate them. The complexes can be ingested by macrophages and neutrophils through Fc receptors and then destroyed. -Antibodies bound to viruses -> block viruses from infecting cells -Antibodies bound to bacteria -> prevent bacterial colonization of the body -Pentameric IgM is effective in agglutinating (clumping) pathogens

How does a secretory piece make IgA more resistant to degradation?

This occurs via transcytosis of IgA into the Mucosal Secretions. IgA binds to poly-Ig receptor on the basolateral face of epithelial cell -> receptor mediated endocytosis of IgA --> IgA is transported to apical face of epithelial cell. The Poly-Ig receptor is cleaved which leaves the dimeric IgA bound to the secretory piece. - The secretory piece makes IgA more resistant to degradation protecting it from the enzymes in mucosal secretions.

Isotype switching is driven by what?

This process is driven by cytokines from T helper cells. -Particular cytokines induce switching to particular isotypes.

What are the effector functions of IgE?

basophil & mast cell activation

Where can you find membrane-bound IgA and what is its structure?

membrane-bound IgA is a bivalent monomer - Plasma cells in lymph nodes and spleen secrete monomeric IgA that is part of the systemic immune system, but plasma cells in these organs make much more IgG than IgA.

What are the effector functions of dimeric IgA?

neutralization, neonatal immunity, (serum monomeric IgA: opsonization)