Structure of Antibody
Antibodies are glycoproteins. The basic functional unit of each antibody is an immunoglobulin (Ig)
monomer (containing only one Ig unit); secreted antibodies can also be dimeric with two Ig units as with
IgA, tetrameric with four Ig units like teleost fish IgM, or pentameric with five Ig units, like mammalian IgM.
Several immunoglobulin domains make up the two heavy chains and the two light chains of an antibody.
The immunoglobulin domains are composed of between 7 (for constant domains) and 9 (for variable
domains) ß-strands. Note: The variable parts of an antibody are its V regions, and the constant part is its
C region.
Immunoglobulin domains: The Ig monomer is a “Y”-shaped molecule that consists of four polypeptide
chains; two identical heavy chains and two identical light chains connected by disulfide bonds. Each chain
is composed of structural domains called immunoglobulin domains. These domains contain about 70-110
amino acids and are classified into different categories (for example, variable or IgV, and constant or IgC)
according to their size and function. They have a characteristic immunoglobulin fold in which two beta
sheets create a “sandwich” shape, held together by interactions between conserved cysteines and other
charged amino acids.
Heavy chain: There are five types of mammalian Ig heavy chain denoted by the Greek letters: α, δ, ε, γ,
and µ. The type of heavy chain present defines the class of antibody; these chains are found in IgA, IgD,
IgE, IgG, and IgM antibodies, respectively. Distinct heavy chains differ in size and composition; α and γ
contain approximately 450 amino acids, while µ and ε have approximately 550 amino acids.
Each heavy chain has two regions, the constant region and the variable region. The constant region is
identical in all antibodies of the same isotype, but differs in antibodies of different isotypes. Heavy chains
γ, α and δ have a constant region composed of three tandem (in a line) Ig domains, and a hinge region
for added flexibility; heavy chains µ and ε have a constant region composed of four immunoglobulin
domains. The variable region of the heavy chain differs in antibodies produced by different B cells, but is
the same for all antibodies produced by a single B cell or B cell clone. The variable region of each heavy
chain is approximately 110 amino acids long and is composed of a single Ig domain.
Light chain: For more details on this topic, see Immunoglobulin light chain. In mammals there are two
types of immunoglobulin light chain, which are called lambda (λ) and kappa (κ). A light chain has two
successive domains: one constant domain and one variable domain. The approximate length of a light
chain is 211 to 217 amino acids. Each antibody contains two light chains that are always identical; only
one type of light chain, κ or λ, is present per antibody in mammals. Other types of light chains, such as
the iota (ι) chain, are found in lower vertebrates like Chondrichthyes and Teleostei.
CDRs, Fv, Fab and Fc Regions: Some parts of an antibody have unique functions. The arms of the Y,
for example, contain the sites that can bind two antigens (in general identical) and, therefore, recognize
specific foreign objects. This region of the antibody is called the Fab (fragment, antigen binding) region. It
Antibodies are glycoproteins. The basic functional unit of each antibody is an immunoglobulin (Ig)
monomer (containing only one Ig unit); secreted antibodies can also be dimeric with two Ig units as with
IgA, tetrameric with four Ig units like teleost fish IgM, or pentameric with five Ig units, like mammalian IgM.
Several immunoglobulin domains make up the two heavy chains and the two light chains of an antibody.
The immunoglobulin domains are composed of between 7 (for constant domains) and 9 (for variable
domains) ß-strands. Note: The variable parts of an antibody are its V regions, and the constant part is its
C region.
Immunoglobulin domains: The Ig monomer is a “Y”-shaped molecule that consists of four polypeptide
chains; two identical heavy chains and two identical light chains connected by disulfide bonds. Each chain
is composed of structural domains called immunoglobulin domains. These domains contain about 70-110
amino acids and are classified into different categories (for example, variable or IgV, and constant or IgC)
according to their size and function. They have a characteristic immunoglobulin fold in which two beta
sheets create a “sandwich” shape, held together by interactions between conserved cysteines and other
charged amino acids.
Heavy chain: There are five types of mammalian Ig heavy chain denoted by the Greek letters: α, δ, ε, γ,
and µ. The type of heavy chain present defines the class of antibody; these chains are found in IgA, IgD,
IgE, IgG, and IgM antibodies, respectively. Distinct heavy chains differ in size and composition; α and γ
contain approximately 450 amino acids, while µ and ε have approximately 550 amino acids.
Each heavy chain has two regions, the constant region and the variable region. The constant region is
identical in all antibodies of the same isotype, but differs in antibodies of different isotypes. Heavy chains
γ, α and δ have a constant region composed of three tandem (in a line) Ig domains, and a hinge region
for added flexibility; heavy chains µ and ε have a constant region composed of four immunoglobulin
domains. The variable region of the heavy chain differs in antibodies produced by different B cells, but is
the same for all antibodies produced by a single B cell or B cell clone. The variable region of each heavy
chain is approximately 110 amino acids long and is composed of a single Ig domain.
Light chain: For more details on this topic, see Immunoglobulin light chain. In mammals there are two
types of immunoglobulin light chain, which are called lambda (λ) and kappa (κ). A light chain has two
successive domains: one constant domain and one variable domain. The approximate length of a light
chain is 211 to 217 amino acids. Each antibody contains two light chains that are always identical; only
one type of light chain, κ or λ, is present per antibody in mammals. Other types of light chains, such as
the iota (ι) chain, are found in lower vertebrates like Chondrichthyes and Teleostei.
CDRs, Fv, Fab and Fc Regions: Some parts of an antibody have unique functions. The arms of the Y,
for example, contain the sites that can bind two antigens (in general identical) and, therefore, recognize
specific foreign objects. This region of the antibody is called the Fab (fragment, antigen binding) region. It
