Ch. 5: Antibody Structure and Function Flashcards
What are antibodies
antibodies are immunoglobulins that play a key role in the humoral immune response
Where is the highest concentration of antibodies found
in the serum
What percentage of antibodies are glycoproteins
80-95%
What percentage of antibodies are carbohydrates
1-15%
What are the five major classes of antibodies
- IgG
- IgA
- IgM
- IgE
- IgD
(GAMED)
What is the main function of antibodies
To bind foreign antigens and neutralize or eliminate foreign invaders
What is the basic structure of an immunoglobulin
a four chain polypeptide structure
What are the two types of peptide chains in an immunoglobulin
Heavy (H) and Light (L) chains
How many heavy and light chains does an immunoglobulin have
two heavy and two light chains
What holds the chains of an immunoglobulin together
Noncovalent forces and S=S disulfide bonds
What is the function of the variable region (amino-terminal end) of an immunoglobulin
allows for specific binding to a particular antigen
Where are the constant regions located in an immunoglobulin, and what is their function
The constant regions (carboxy-terminal end) are responsible for the biological functions of the antibody.
How many constant regions do immunoglobulins have
3-4 constant regions
How are heavy chains in immunoglobulins denoted
by greek letters specific to each Ig class
What domains do heavy chains have
- one variable domain (VH)
- 3-4 constant domains (CH1, CH2, CH3, CH4)
What are the two types of light chains in immunoglobulins?
Kappa (κ) and Lambda (λ)
Can an immunoglobulin molecule have both types of light chains
No, a single immunoglobulin molecule has only one type of light chain
What is the function of the hinge region in immunoglobulins
- Provides flexibility of Fab arms
- assists in initiation of complement cascade and Fc receptor binding
What domains do light chains contain
- 2 variable light chain domains (VL)
- 2 constant light chain domains (CL)
Where is the hinge region located
between the CH1 and CH2 regions of a Ig
Where is the carbohydrate portion located
located between the CH2 domains
What is the function of the carbohydrate portion of immunoglobulins
- Increases solubility
- provides protection against degradation of molecule
- enhances functional activity of the Fc domain
What are the two major structural regions of an antibody?
- Fab region
- Fc region
What does the Fab region do and what is it made of
- Antigen binding site
- consists of light chain and half of the heavy chain
What does the Fc region do and what is it made of
- interacts with host systems, promotes clearance of antigens and activates immune responses
- consists of half of the heavy chain
What happens when an antibody is treated with papain
- Produces two Fab fragments (antigen-binding, no effector function)
- Produces one Fc fragment (mediates effector function, cannot bind antigen
What happens when an antibody is treated with pepsin?
- Produces Fab fragments (two antigen-binding sites held together)
Produces Fc fragment (non-functional pieces)
What are isotypes in immunoglobulins
Unique amino acid sequences common to all Igs of a given class or subclass, located in the constant region of heavy chains
List the heavy chains associated with each Ig class
IgG: γ (gamma)
IgM: μ (mu)
IgA: α (alpha)
IgD: δ (delta)
IgE: ε (epsilon)
What is the clinical significance of isotypes
Used for Ig quantitation, diagnosing immunodeficiency diseases, and classifying B cell leukemias
What are allotypes in immunoglobulins
Minor variations in amino acid sequences found in some individuals of the same species, inherited and individual-specific
Where are allotypes located
In the constant regions of the four IgG subclasses, one IgA subclass, and the λ light chain
What is the clinical significance of allotypes
Used in bone marrow graft monitoring, forensics, and paternity testing
What are idiotypes in immunoglobulins
Variations in the variable regions that give antibodies their specificity to a particular antigen
Where are idiotypes located
In the variable heavy (VH) and variable light (VL) chain regions
What is the clinical significance of idiotypes
Used in immune response regulation, vaccines, and B cell tumor treatments
Which immunoglobulin is the predominant antibody in humans
IgG
What percentage of total serum immunoglobulin does IgG constitute
70-75%
Which immunoglobulin has the longest half-life, and how long is it
IgG, with a half life of 23 days
What type of heavy chains does IgG have
Gamma (γ) heavy chains
Does IgG production require T-cell help
yes
How many subclasses of IgG exist, and what are they
- IgG1
- IgG2
- IgG3
- IgG4
How do IgG subclasses differ from each other?
They have slight differences in constant region amino acid sequences, number, and position of disulfide bridges
Which IgG subclasses respond to protein antigens?
IgG1 and IgG3
Which IgG subclasses respond to polysaccharide antigens?
IgG2 and IgG4
What are the major functions of IgG?
Complement activation, antigen opsonization, ADCC, toxin and virus neutralization, newborn immunity, and precipitation/agglutination (in vitro).
Which IgG subclass is the most efficient at complement activation
IgG3, followed by IgG1
What is antibody-dependent cell-mediated cytotoxicity (ADCC)?
A process where IgG binds to target cells, allowing NK cells and other immune cells to destroy them.
Which immunoglobulin is the only one that can cross the placenta
IgG
Why is IgG important for newborn immunity?
It provides passive immunity by crossing the placenta and protecting the infant after birth
What Ig is the largest of all the classes
IgM
(M for mega fatty)
What is another name for IgM
macroglobulin
What percentage of total serum immunoglobulins does IgM account for?
5-10%
What is the half-life of IgM
6 days
In what two forms can IgM exist?
- Membrane-bound monomer (on B cell surface)
- Pentamer (in blood)
What structure holds the pentameric form of IgM together
The J chain that forms disulfide bonds
How many antigen-binding sites does IgM have in its pentameric form?
10 antigen-binding sites
Describe the structural arrangement of IgM.
It has a star-like shape with Fc regions in the center and Fab arms extended outward
