_ANTIGEN
_
Antigen
is a substance (molecule) that reacts with its specific
free antibody, and/or antigen binding receptor on lymphocytes.
Immunogen is a substance (molecule) that induce an immune response.
All immunogens are antigen but all antigens are not immunogen. The terms antigen and immunogen are used interchangeably.
Antigenicity is the ability of an antigen to interact specifically with its specific free antibody and/or with antigen-binding receptors on lynmphocytes.
Immunogenicity is the ability to induce a humoral and/or cell-mediated immune response. B and T cells recognize epitopes or antigenic determinants on an immunogen:
B cells + antigen -* Plasma cells + memory B cells
Antibody -T cells + antigen--4 Effector + memory Th or Tc cells.
Typical immunogens include pathogenic microorganisms (e.g. bacteria, viruses, and parasites), foreign tissue graft, proteins in pollen, grass, or food. A microorganism may contain several to multitude immunogens.
Antigenicity/immunogenicity:
Properties and features of antigens that determine immunogenicity i.e. ability to induce immune response:
1. Foreignness. For immunogenicity, the molecule must be recognised as 'nonself'. A molecule recognised as 'self' is not immunogenic to the individual. Sometimes, a component of an individual behaves as nonself and acts as an antigen for himself and is called "autoantigen".
2. Molecular size-The most potent immunogens are proteins with molecular weight above 100,000. Molecules of less than 10,000 molecular weight are weakly immunogenic, e.g. insulin (mol. wt 5808), glucagon (mol. wt. 3800). Small molecules like amino acids are nonimmunogenic. Certain small molecules, e.g. heptens become immunogen only when linked to a carrier protein.
3. Chemical complexity. Most immunogens are proteins. Nucleoproteins are strong immunogens. Glycoproteins like A and B blood group and Rh antigens are immunogens. Lipoproteins and lipopolysaccharides can be immunogen. Polypeptides like insulin are usually weakly immunogenic. Most lipids are nonimmunogenic. Amino acids are nonimmunogenic.
Immunogen is a substance (molecule) that induce an immune response.
All immunogens are antigen but all antigens are not immunogen. The terms antigen and immunogen are used interchangeably.
Antigenicity is the ability of an antigen to interact specifically with its specific free antibody and/or with antigen-binding receptors on lynmphocytes.
Immunogenicity is the ability to induce a humoral and/or cell-mediated immune response. B and T cells recognize epitopes or antigenic determinants on an immunogen:
B cells + antigen -* Plasma cells + memory B cells
Antibody -T cells + antigen--4 Effector + memory Th or Tc cells.
Typical immunogens include pathogenic microorganisms (e.g. bacteria, viruses, and parasites), foreign tissue graft, proteins in pollen, grass, or food. A microorganism may contain several to multitude immunogens.
Antigenicity/immunogenicity:
Properties and features of antigens that determine immunogenicity i.e. ability to induce immune response:
1. Foreignness. For immunogenicity, the molecule must be recognised as 'nonself'. A molecule recognised as 'self' is not immunogenic to the individual. Sometimes, a component of an individual behaves as nonself and acts as an antigen for himself and is called "autoantigen".
2. Molecular size-The most potent immunogens are proteins with molecular weight above 100,000. Molecules of less than 10,000 molecular weight are weakly immunogenic, e.g. insulin (mol. wt 5808), glucagon (mol. wt. 3800). Small molecules like amino acids are nonimmunogenic. Certain small molecules, e.g. heptens become immunogen only when linked to a carrier protein.
3. Chemical complexity. Most immunogens are proteins. Nucleoproteins are strong immunogens. Glycoproteins like A and B blood group and Rh antigens are immunogens. Lipoproteins and lipopolysaccharides can be immunogen. Polypeptides like insulin are usually weakly immunogenic. Most lipids are nonimmunogenic. Amino acids are nonimmunogenic.
_ 4.
Epitope
(Antigenic determinant). Epitope is the specific antigenic site of the antigen molecule. An epitope is the small chemical group on the antigen
molecule that elicits and reacts with the antibody or effector T cell. An epitope is a peptide of just 4 or 5 amino
acids, or a monosaccharide in size. An antigen molecule may have one or more epitopes (Fig. 63.3).
5. Amount of Antigen. Immune response depends on amount of antigen.
6. Interval between doses.
7. Route of Administration of the antigen, e.g. epitope of the antigen molecule may be destroyed if given orally.
8. Genetic Makeup of the Host : An example is the alloantigen which is the antigen present in non-identical individuals of the same species, e.g. Blood group antigens.
Haptens are usually small molecules, e.g. penicillin. Haptens are antigenic but not immunogenic. Haptens are not immunogenic because they cannot activate helper T cells as they are not polypeptides. Hapten also cannot activate B cells by themselves as they are not multivalent macromolecules. Hapten becomes immunogen when combines with a carrier protein. Hapten-carrier protein complex triggers production of anti-hapten antibody with which the hapten reacts specifically.
5. Amount of Antigen. Immune response depends on amount of antigen.
6. Interval between doses.
7. Route of Administration of the antigen, e.g. epitope of the antigen molecule may be destroyed if given orally.
8. Genetic Makeup of the Host : An example is the alloantigen which is the antigen present in non-identical individuals of the same species, e.g. Blood group antigens.
Haptens are usually small molecules, e.g. penicillin. Haptens are antigenic but not immunogenic. Haptens are not immunogenic because they cannot activate helper T cells as they are not polypeptides. Hapten also cannot activate B cells by themselves as they are not multivalent macromolecules. Hapten becomes immunogen when combines with a carrier protein. Hapten-carrier protein complex triggers production of anti-hapten antibody with which the hapten reacts specifically.
_Processing and Presentation of Antigens
_ For recognition of a foreign protein
antigen by a T cell it is degraded
into antigenic peptides by sequence of events within the cell. The degraded peptides then form
complexes with class I or class II
MHC molecules in different compartments within the cytoplasm and are then transported to the surface membrane of the cell where they are displayed.
This is called antigen processing and presentation.
1. Exogenous antigen is produced outside the host such as in bacteria and is internalized by antigen presenting cells by endocytosis or phagocytosis. Antigen presenting cells (APC) are macrophages, dendritic cells, Langerhans cells and B cells. Within APC the antigen is degraded into peptides which is complexed with class II MHC molecule and is displayed qn the cell surface, and is recognized by CD4 T cell receptor (TCR). Processing occurs in endocytic pathway (class II MHC pathway).
2. Endogenous antigen is produced within the cell itself (e.g. virus-infected cell) of the host. The virus is degraded into peptide within the cell cytosole. The peptide is complexed with class I MHC molecule and is displayed on the cell surface, and is recognized by CD8 T cell receptor (TCR). Processing occurs in cytosolic pathway (class I MHC pathway).
1. Exogenous antigen is produced outside the host such as in bacteria and is internalized by antigen presenting cells by endocytosis or phagocytosis. Antigen presenting cells (APC) are macrophages, dendritic cells, Langerhans cells and B cells. Within APC the antigen is degraded into peptides which is complexed with class II MHC molecule and is displayed qn the cell surface, and is recognized by CD4 T cell receptor (TCR). Processing occurs in endocytic pathway (class II MHC pathway).
2. Endogenous antigen is produced within the cell itself (e.g. virus-infected cell) of the host. The virus is degraded into peptide within the cell cytosole. The peptide is complexed with class I MHC molecule and is displayed on the cell surface, and is recognized by CD8 T cell receptor (TCR). Processing occurs in cytosolic pathway (class I MHC pathway).
_NOTE
_1.Superantigens.
Superantigens activate a large number of helper
T cells (CD4 T cells) in contrast to the usual antigen which
activate one or a few helper T cells. It leads to release of large
amounts of
cytokines that take part in pathogenesis of
diseases caused by organisms with superantigens
to a large extent. Certain proteins particularly toxic shock syndrome
toxin and enterotoxins of staphylococcus, and pyrogenic exotoxin A of
group
A streptococcus are superantigens.
2. Adjuvant. Adjuvant is a substance that nonspecifically enhances the immune response of an antigen by releasing the immunogen slowly from depot after injection. Some human vaccines contain adjuvants such as aluminum hydroxide or aluminum phosphate in diphtheria toxoid.
2. Adjuvant. Adjuvant is a substance that nonspecifically enhances the immune response of an antigen by releasing the immunogen slowly from depot after injection. Some human vaccines contain adjuvants such as aluminum hydroxide or aluminum phosphate in diphtheria toxoid.