Antigenic Specificity: Epitopes
Antigenic specificity of the antigen depends on antigenic determinants or
epitopes.
B and T-Cell Receptors
One of the major routes by which a cell interprets its surroundings is through the binding of
signaling molecules to cell-associated receptor proteins. A molecule that binds to a receptor
is a ligand. Noncovalent binding of a ligand to its receptor may induce alterations in the
receptor itself, in its polymerization state, and/or in the environment of that receptor.
These changes act to transmit or transduce the ligand-binding signal into the interior of the
cell, leading to alterations in cellular functions.
Receptor-Ligand Interactions
The surface of a receptor molecule binds to its complementary ligand surface by the same
types of noncovalent chemical linkages that enzymes use to bind to their substrates. These
include hydrogen and ionic bonds, and hydrophobic and van der Waals interactions. The
key to a meaningful receptor-ligand interaction is that the sum total of these bonding
interactions holds the two interacting surfaces together with sufficient binding energy, and
for sufficient time, to allow a signal to pass from the ligand to the cell bearing the receptor.
Because these noncovalent interactions are individually weak, many such interactions are
required to form a biologically significant receptor-ligand connection.
Furthermore, since each of these noncovalent interactions operates only over a very short
distance-generally about 1 Angstrom, a high-affinity receptor ligand interaction depends on
a very close “fit,” or degree of complementarity, between the receptor and the ligand.
Epitopes
An epitope is defined as the immunologically active region of an immunogen/ antigen that
binds to antigen-specific membrane receptors on lymphocytes or secreted antibodies.
The interaction between cells of the immune system and antigens takes place at many
levels and the complexity of any antigen is mirrored by its epitope. There are two types of
epitopes: T-cell epitopes and B-cell epitopes.
T cell epitopes
The peptides that can be recognized by the T-cell receptors after a particular antigen has
been intracellularly processed, bound to at least one MHC molecule and expressed on the
surface of the antigen presenting cell as a MHC-peptide complex, are called a T-cell epitope.
T cells recognize amino acids in proteins but do not recognize polysaccharide or nucleic acid
antigens. This is the reason why polysaccharides are considered as T independent antigens
and proteins as T-dependent antigens. The primary sequence of amino acids in proteins
determines the antigenic determinants recognized by T cells. Free peptides are not
Antigenic specificity of the antigen depends on antigenic determinants or
epitopes.
B and T-Cell Receptors
One of the major routes by which a cell interprets its surroundings is through the binding of
signaling molecules to cell-associated receptor proteins. A molecule that binds to a receptor
is a ligand. Noncovalent binding of a ligand to its receptor may induce alterations in the
receptor itself, in its polymerization state, and/or in the environment of that receptor.
These changes act to transmit or transduce the ligand-binding signal into the interior of the
cell, leading to alterations in cellular functions.
Receptor-Ligand Interactions
The surface of a receptor molecule binds to its complementary ligand surface by the same
types of noncovalent chemical linkages that enzymes use to bind to their substrates. These
include hydrogen and ionic bonds, and hydrophobic and van der Waals interactions. The
key to a meaningful receptor-ligand interaction is that the sum total of these bonding
interactions holds the two interacting surfaces together with sufficient binding energy, and
for sufficient time, to allow a signal to pass from the ligand to the cell bearing the receptor.
Because these noncovalent interactions are individually weak, many such interactions are
required to form a biologically significant receptor-ligand connection.
Furthermore, since each of these noncovalent interactions operates only over a very short
distance-generally about 1 Angstrom, a high-affinity receptor ligand interaction depends on
a very close “fit,” or degree of complementarity, between the receptor and the ligand.
Epitopes
An epitope is defined as the immunologically active region of an immunogen/ antigen that
binds to antigen-specific membrane receptors on lymphocytes or secreted antibodies.
The interaction between cells of the immune system and antigens takes place at many
levels and the complexity of any antigen is mirrored by its epitope. There are two types of
epitopes: T-cell epitopes and B-cell epitopes.
T cell epitopes
The peptides that can be recognized by the T-cell receptors after a particular antigen has
been intracellularly processed, bound to at least one MHC molecule and expressed on the
surface of the antigen presenting cell as a MHC-peptide complex, are called a T-cell epitope.
T cells recognize amino acids in proteins but do not recognize polysaccharide or nucleic acid
antigens. This is the reason why polysaccharides are considered as T independent antigens
and proteins as T-dependent antigens. The primary sequence of amino acids in proteins
determines the antigenic determinants recognized by T cells. Free peptides are not
