Immunology: Antibody Structure and Function

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Questions and Answers

What is the significance of the antigen-binding site formed by the heavy and light chains of an immunoglobulin monomer?

The antigen-binding site facilitates the specific recognition and binding of antibodies to their respective antigens.

What forms the antigen-binding site in an immunoglobulin molecule?

The antigen-binding site is formed by the combination of a light chain variable domain (VL) and a heavy chain variable domain (VH).

How many domains are typically found within the light and heavy chains of an immunoglobulin molecule?

Each light and heavy chain generally consists of multiple domains, approximately 110 amino acids each.

What determines the immunoglobulin class or subclass in an antibody?

The immunoglobulin class or subclass is determined by the type of heavy chain isotype present in the antibody. Signup and view all the answers

How does the variability of amino acid sequences in immunoglobulin chains contribute to immune diversity?

The variability in the VL and VH domains allows for the recognition of a vast array of different epitopes, enhancing immune diversity. Signup and view all the answers

What are the two types of light chains found in immunoglobulins, and how are they genetically encoded?

The two types of light chains are kappa (κ) and lambda (λ), which are encoded on chromosomes 2 and 22, respectively. Signup and view all the answers

What structural feature distinguishes the heavy chains of μ and ε from those of δ, Ƴ, and α isotypes?

μ and ε heavy chains contain four constant domains (CH4), while δ, Ƴ, and α chains have three constant domains (CH1, CH2, CH3). Signup and view all the answers

What is the role of disulfide bonds in the structure of immunoglobulins?

Disulfide bonds link the heavy and light chains together, stabilizing the overall structure of the immunoglobulin monomer. Signup and view all the answers

What are the principal fragments produced when immunoglobulin is cleaved by papain, and what do they contain?

Papain cleavage produces two Fab fragments and one Fc fragment; each Fab contains VL, VH, and CH1, while Fc contains CH2, CH3, and sometimes CH4. Signup and view all the answers

Describe the difference between Fab and Fd fragments in terms of structure.

Fab consists of both the heavy chain (VH, CH1) and light chain (VL, CL), whereas Fd contains only the heavy chain portion (VH, CH1). Signup and view all the answers

Explain the term 'iso-type' in the context of immunoglobulins.

Iso-types refer to the genetically different forms of heavy and light chains that define different classes of immunoglobulins. Signup and view all the answers

Identify one key function of immunoglobulin cleavage fragments.

Immunoglobulin cleavage fragments can retain the ability to bind to specific antigens, thus aiding in immune recognition. Signup and view all the answers

What role does the hinge region play in the structure of immunoglobulins?

The hinge region provides flexibility to the immunoglobulin molecule, allowing for better antigen binding and interaction. Signup and view all the answers

What are the enzymatic cleavage products of immunoglobulin when acted upon by pepsin?

Pepsin cleavage produces F(ab')2 fragments, which consist of two Fab fragments linked by remnants of the heavy chain. Signup and view all the answers

In immunoglobulin monomers, what is the composition of the chains, and how does this influence their function?

An immunoglobulin monomer comprises two identical light chains and two identical heavy chains, influencing its ability to bind specifically to antigens. Signup and view all the answers

What is the significance of the variability in the constant domains of immunoglobulins?

Variable constant domains reflect the limited variability across different isotypes, contributing to the specific roles of antibodies in the immune response. Signup and view all the answers

What role do hypervariable regions play in the structure and function of antibodies?

Hypervariable regions allow antibodies to bind to specific antigens; they form the antigen-binding site and are crucial for antigen specificity. Signup and view all the answers

Explain the difference between isotypes, allotypes, and idiotypes in immunoglobulins.

Isotypes refer to the different classes of immunoglobulins defined by heavy and light chain types, allotypes are variants of the same isotype within a species, and idiotypes refer to unique antigen-binding sites on the antibodies. Signup and view all the answers

Describe how dimeric IgA is formed and its significance in mucosal immunity.

Dimeric IgA is formed by the addition of a J chain to two monomeric IgA antibodies, which allows effective transport to mucosal surfaces for immune protection. Signup and view all the answers

What is the significance of the complementarity-determining regions (CDRs) in the context of antigen recognition?

CDRs are vital for antigen recognition because their variability allows antibodies to interact with diverse antigens with high specificity. Signup and view all the answers

How do structural differences in heavy and light chains determine immunoglobulin isotypes?

Immunoglobulin isotypes are determined by structural differences in the constant regions of heavy and light chains. Signup and view all the answers

What are the implications of an antibody acting as an antigen when introduced into a different species?

When an antibody from one species is injected into another, it is recognized as foreign, inducing the formation of anti-isotypic antibodies in the recipient. Signup and view all the answers

Discuss the role of accessory molecules in the transport of dimeric IgA.

Accessory molecules, formed from specialized receptors on epithelial cells, facilitate the transport of dimeric IgA to mucosal surfaces. Signup and view all the answers

What determines the immunological specificity of an antibody?

The immunological specificity of an antibody is determined by its hypervariable regions, which define the unique antigen-binding site. Signup and view all the answers

Study Notes

Immune Response

Primary and secondary immune responses involve antigens, immunogens, adjuvants, and haptens.
Immunoglobulins have structure, classes, subclasses, hypervariable regions, isotypes, allotypes, and idiotypic variations.
Subsets of T cells (T-helper, T-killer, suppressor cells) and B cells exist.
T and B cell receptors process and present antigens.
T and B cell interactions involve cytokines and co-stimulatory molecules like lymphokines and interleukins.
The complement system is also involved.

Primary and Secondary Humoral Immune Responses

Primary response is characterized by a smaller magnitude and usually IgM > IgG antibodies after the first infection.
Secondary response involves a greater magnitude, relative increase in IgG (often IgA and sometimes IgE), higher average affinity antibodies (affinity maturation) after repeat infection.
Primary responses involve all antigens, while secondary responses mainly to protein antigens.
A primary response involves activation and differentiation of naive B cells into antibody-secreting cells.
A secondary response involves memory B cells leading to production of specific antibodies than the primary response.

Antigens

Antigens are substances that bind specifically to an antibody or T-cell receptor and are often used as a synonym for immunogens.
Antigen processing involves degrading antigens into antigenic peptides displayed on MHC molecules on antigen-presenting cells (APCs).
Professional APCs include macrophages, dendritic cells, and B cells.
Nonprofessional APCs include thymic epithelial cells and vascular endothelial cells.

Adjuvants

Adjuvants are bacterial components or substances, often in an oil medium, that increase the effectiveness of vaccinations.
They cause mild inflammation and attract phagocytes, accelerating activation and antigen presentation to T cells.
Some vaccine components act as their own adjuvant.
Alum and BCG are examples of adjuvants.

Haptens

Haptens are small chemical groups that can bind to antibodies but cannot stimulate an adaptive immune response alone.
They require attachment to a larger macromolecule (carrier) for an immune response.
Examples include dinitrophenol (DNP).

Tolerogens

Tolerogens induce adaptive immune unresponsiveness.
Exposure to a tolerogen leads to a diminished response rather than an enhanced one, unlike immunogens.
Tolerance to self-molecules develops during immune repertoire development.
Non-self antigens are recognized as foreign.

Immunoglobulins

Immunoglobulins are synthesized and secreted by plasma cells, which are terminally differentiated B cells.
The term antibody is used for an immunoglobulin with specificity for an epitope of antigens.
Antibodies bind to antigens, neutralize them, or tag them for destruction.
Antibodies are important components of the humoral immune system.

Immunoglobulin Monomer Structure

Immunoglobulin monomers contain two identical heavy chains and two identical light chains, connected by disulfide bonds.
Each chain has a variable domain and one or more constant domains.
Light chains are κ (kappa) or λ (lambda) and heavy chains are μ, delta, gamma, epsilon, and alpha (for example).
Variable regions are highly variable between different immunoglobulins.

MHC Molecules

Class I MHC molecules display peptides derived from proteins in the cytosol for recognition by CD8+ T cells.
Class II MHC molecules display peptides derived from extracellular proteins for recognition by CD4+ T cells.

T Cell Receptor (TCR)

TCRs, including a and β chains, are involved in antigen binding.
TCRs have variable and constant regions similar to antibodies.

Immunoglobulin Isotypes, Allotypes, and Idiotypes

Isotypes are based on differences in heavy chain constant regions.
Allotypes are based on variations in constant regions due to genetic variation of the same gene.
Idiotypes are the unique variable region sequences of an immunoglobulin, which are involved in antigen-binding sites.

Complement System

The complement system involves plasma and cell surface proteins that interact to opsonize microbes, recruit phagocytes to infection sites, and directly kill microbes in some cases.
The classical, alternative, and lectin pathways lead to inflammation and opsonization, and formation of a membrane attack complex(MAC) to lyse microbes.
Key components include C1q, C3, C3a and C3b, C5, C6, C7, C8, C9.
Proteins like MBL & ficalins are part of the lectin pathway.

Cytokines

Cytokines are low-molecular-weight protein messengers that regulate and coordinate immune responses.
They influence growth, differentiation, inflammation, and repair.

T Cell Subsets

CD4+ T cells are helper T cells that secrete cytokines impacting other immune cells.
CD8+ T cells (CTLs) recognize and kill virus/bacteria-infected cells and cancer cells.
Other T cells exist (NKT cells, MAIT cells, γδ T cells)
Cytokines regulate the development of Th1, Th2, and Th17 cells.

B Cell Subsets

Follicular B cells, marginal zone B cells and B-1 cells contribute to humoral immunity through various interactions and antibody production, depending on T cell dependency.
T cell independent development occurs in the spleen, peritoneal cavity and mucosal sites.

Interactions between B and T Cells

Direct interaction between B and T cells often requires co-stimulatory molecules.
These interactions lead to B cell proliferation and differentiation into plasma cells.
Antigen processing and presentation are essential for these interactions.

Receptors

There are many types of receptors involved: complement receptors, cytokine receptors, BCRs, TCRs.
They are each specific in their function and activation
Specific receptors are located on the surface of different immune cells.

B cells and T cells

B cells and T cells cooperate and work together to produce a high-quality, specific, adaptive immune response
Cell surface molecules regulate the interaction between B and T cells.

Antigen-Presenting Cells (APCs):

APCs process and present antigens.
APCs also express costimulatory molecules to activate T lymphocytes (CD28).

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Description

Explore the critical features of immunoglobulin structure in this quiz, including the formation of antigen-binding sites and the significance of various chains. Understand how structural variations contribute to immune diversity and the roles of different antibody fragments.