Antigens are substances that induce an immune response and react with the products of that response. They can be classified as exogenous, endogenous, autoantigens, or tumor antigens based on their origin. Antigens have immunogenic and antigenic properties that allow them to stimulate immune responses and bind antibodies. Epitopes are the specific sites on antigens that are recognized by immune cells. Antibodies are Y-shaped proteins produced in response to antigens that help eliminate pathogens in the body. The main antibody classes are IgG, IgM, IgA, IgE, and IgD, which have different structures and functions.

1. Antigen

2. Concept of Antigen
• Antigens are substances that induce a specific
immune response and subsequently react with
the products of a specific immune response.

3. • An antigen is a molecule that stimulates an
immune response.
• The word originated from the notion that they
can stimulate antibody generation. We now
know that the immune system does not only
consist of antibodies.
• The modern definition encompasses all
substances that can be recognized by the
adaptive immune system.

4. • Tolerogen - An antigen that invokes a
specific immune non-responsiveness due
to its molecular form. If its molecular form
is changed, a tolerogen can become an
immunogen.

5. • Allergen - An allergen is a substance that
causes the allergic reaction. The
(detrimental) reaction may result after
exposure via ingestion, inhalation,
injection or contact with skin.

6. Antigens can be classified in order of their origins
• Exogenous antigens
Exogenous antigens are antigens that have
entered the body from the outside, for
example by inhalation, ingestion, or injection.
By endocytosis or phagocytosis, these
antigens are taken into the antigen-
presenting cells (APCs) and processed into
fragments.

7. • Endogenous antigens
Endogenous antigens are antigens that have
been generated within the cell, as a result of
normal cell metabolism, or because of viral or
intracellular bacterial infection.

8. • Autoantigens
An autoantigen is usually a normal
protein or complex of proteins (and
sometimes DNA or RNA) that is
recognized by the immune system of
patients suffering from a specific
autoimmune disease.

9. These antigens should under normal
conditions not be the target of the immune
system, but due to mainly genetic and
environmental factors the normal
immunological tolerance for such an antigen
has been lost in these patients.

10. • Tumor antigens
Tumor antigens are those antigens that are
presented by the MHC I molecules on the
surface of tumor cells. These antigens can
sometimes be presented only by tumor cells
and never by the normal ones. In this case,
they are called tumor-specific antigens
(TSAs) and typically result from a tumor
specific mutation.

11. Characteristics of Antigen
★ Immunogenicity
The capacity to stimulate the production
of antibodies or cell-mediated immune
responses.

12. ★ Antigenicity
The ability to bind antibody.
♣ Complete antigen
♣ Incomplete antigen, also known as
hapten

13. Incomplete antigens have antigenic
determinants, but cannot induce immune
responses because they lack one or more of
the important attributes needed for this
function (one example of an incomplete
antigen is a hapten, which is an artificial
monovalent epitope)

14. Properties of antigen
• Foreignness
• Specificity
• High molecular weight: Min 750 Da
• Structural Stability
• Degradability
• Route of administration
• Host genetics
• Dose of Antigens

15. Molecule of Hemoglobin

16. Antigenic epitopes
Epitope,or,Antigenic determinants, are
the portions of antigen molecules that
physically interact with paratopes
(combining sites) of immune response
molecules and therefore actually
"determine" antigen specificity
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17. Antigenic epitopes

18. Types of Epitopes
1. Linear epitopes
♣ continuous and found in
polysaccharides as well as in both native
(nondenatured) and denatured proteins,
especially fibrillar proteins.
♣ specificity depends upon primary
sequence.
♣ typical size is 5-6 subunits in length.

19. 2. Conformational epitopes
♣ Discontinuous (involve multiple subunits,
often located far apart in the primary
sequence of the antigen molecule) and
are thus found only in native (globular)
proteins.

20. ♣ Specificity depends upon conformation, or
three-dimensional shape, which is a
combination of tertiary and quaternary
structure ... supported by primary and
secondary structure, of course.

21. ♣ Typical size is hard to pinpoint, but
sequences of up to 16 amino acids in
certain protein antigens have been shown
to interact with their complementary
paratope.

22. Two different epitopes
• B cell epitope, a
portion of antigen
molecule that is
recognized by B cell
receptors.
• T cell epitope, the
region of antigen
molecules that are
recognized by T cell
receptors.

23. T cell epitope and B cell epitope

25. Classification of Antigens
• Thymus-dependent antigen(TD-Ag)
• Thymus-independent antigen(TI-Ag)

26. Superantigen
• Molecules that are potent T lymphocyte
mitogens and simultaneously bind to class
II MHC molecules. They are often
associated with staphylococcal products
and are involved in enterotoxemias and
toxic shock syndrome in humans.

27. • Superantigens (SAgs) are secreted
proteins (exotoxins) that exhibit highly
potent lymphocyte-transforming
(mitogenic) activity directed towards T
lymphocytes.

28. • Compared to a normal antigen-induced T-
cell response where 0.001-0.0001% of the
body’s T-cells are activated, SAgs are
capable of activating up to 20% of the
body’s T-cells. This causes a massive
immune response that is not specific to
any particular epitope on the SAg.

29. Conventional Ag

31. Mitogen
• An agent that induces mitosis.
Here means to activate T cells and/or B
cells without help from APCs.
• Lectin, for example, concanavalin A
(ConA).
• LPS(lipopolysaccharide)
• Staphylococcal protein A(SPA)

32. Adjuvant
• Adjuvant: The Latin "adjuvans" means to
help, particularly to reach a goal.
• An adjuvant is a substance that helps and
enhances the pharmacological effect of a
drug or increases the ability of an antigen to
stimulate the immune system.

33. Classification of Adjuvant
• Freund’s adjuvant
♥ Complete Freund’s adjuvant(CFA)
♥ Incomplete Freund’s adjuvant(IFA)
• Liposome
• Inorganic compound
• Cytokine
• Biodegradable nanoparticles

34. Mechanisms of adjuvants
• Prolonged persistence of immunogen
molecules at the site of injection.
• Enhancement of co-stimulatory signals.
• Induction of granuloma formation.
• Stimulation of lymphocyte proliferation in a
non-specific manner.

35. Other antigens
1 Heterophilic antigen
A kind of common antigen, existing in
human, animals, and microbes.
e. g. Fossman antigen.
2 Xenogenic antigen
This antigen comes from different genus
and generic. e. g. pathogenic antigen.

36. 3 Allogenic antigen
The specific antigen exists in different
individuals. Blood type antigens
4 Autoantigen
A pathological term.
BUT, sperm antigen

37. 5 Idiotypic antigen
An antibody molecule is some sort of
foreign molecule when generated in
animal body. Such that immune
system recognizes it as Antigen,
which is known as Idiotypic antigen.

39. Antibody
•Antibody is also termed as Immunoglobulin
•Antibody is globulin (gamma) fraction of serum
proteins
•Antibodies are generated by the immune
system of the body against a specific foreign
antigen
•Antibodies main are component of Humoral
Immunity (HI)
•Rodney Porter first described the detail
structure of antibody molecule and awarded the
Nobel Prize in 1972.

40. Structure of
Immunoglobulin

41. Immunoglobulin Classes
• Ig class Heavy chain Light chain
• IgM µ (mu) κ (kappa) or λ (lambda)
• IgG γ (gamma) κ (kappa) or λ (lambda)
• IgA α (alpha) κ (kappa) or λ (lambda)
• IgE ε (epsilon) κ (kappa) or λ (lambda)
• IgD δ (delta) κ (kappa) or λ (lambda)

42. The major Immunoglobulin Classes
Properties IgG IgM IgA IgE IgD
Mol wt (kDa) 160 900 360 200 160-180
% Carbohydrate 3 12 7 12 12
Electrophoretic
mobility
γ β β - γ β - γ γ
Heavy chain γ μ α ε δ
Heavy chain
domains
4 5 4 5 4
Subclasses 4 None 2 None None
Half life (days) 21 5 6 2 3
Antigen binding
Valency
2 5(10) 2,4 2 2

43. Immunoglobulin Structure
• Each light chain contains about 214 amino acids in two domains.
• The heavy chains of IgG contain about 445 amino acids in 4
domains.
• The variation in amino acid sequence is largely restricted to three
smaller regions within the entire variable region, these regions are
known as hypervariable regions.
• Between the hypervariable regions, the amino acid sequences are
relatively constant and termed as framework regions.
• The hinge region consists of about 12 amino acids located
between CH1 and CH2 domains. The hinge region contains many
hydrophilic and proline residues. The hydrophilic residues make
the peptide chains unfold and thus make this region readily
accessible to proteolytic enzymes. The proline residues make the
chain flexible. This is the region, where papain and pepsin act. This
region also contains all the interchain disulfide bonds.

44. IgG
• Molecular wt: 160 kDa
• Secrets as secondary antibody
• Major secretory antibody in animals
• Found in highest concentration in blood
• Subclasses: IgG1, IgG2, IgG3 and IgG4

45. IgM
Cµ4
Cµ3
Cµ2
C
C
C
µ
4
Cµ3
Cµ2
C
C
Cµ 4
Cµ3
Cµ2
C
C
Cµ4
Cµ3Cµ2
C
C
Cµ4
Cµ3
Cµ2
C
C
•IgM is a pentamer.
•It is primary immunoglobulin.
•Available as a second highest
concentration in mammalian
serum
•All the monomers are bind
together by a cysteine-rich
polypeptide called the J-chain
(15 kDa) binds two of the units
to complete the circle.
•Heavy chain of IgM molecule
contains an additional CH4
domain

46. IgA
• IgA is major secretory antibody of non-ruminants
• IgA produced in body surfaces may either pass through
epithelial cells into external secretions or diffuse into blood
stream. Thus, most of the IgA made in the intestinal wall is
carried into the intestinal fluid.
• The IgA is transported through intestinal epithelial cells bound
to a receptor of 71 kDa protein known as the polymeric Ig
receptor (pIgR) or secretory component. Secretory
components binds covalently to IgA dimers to form a complex
molecule called secretory IgA (SIgA) and protects the IgA from
digestion by intertinal proteases.
JC C
S
S
S
S
C
C
S
S
S
S
C
C
s s

47. IgE and IgD
• IgE is a monomer with an extra CH4 domain in their heavy
chains.
• IgE is largely responsible in immunity against parasitic worms.
• They are also responsible in Type-I hypersensitivity reaction.
• IgE molecules generally found bound on the surface of mast
cells and basophils.
• IgD is primarily a B cell receptor.
• IgD molecule does not have CH2 domain. The CH1 and CH3
domain is separated by a long exposed hinge region.
• It does not have any disulfide bonds, because of that IgD is
highly susceptible to proteolytic enzymes.

48. Immunoglobulin variation
• Isotypes: The structural variations that result in the
production of the different Ig classes occur in all animals of a
species. These variants are called classes or isotypes.
• Allotypes: When variation occurs because of the minor
sequence variations between the proteins of different
individuals. These variations, which are inherited, are called
allotypes.
• Idiotypes: The structural variations in proteins is unique to
the antigen-binding proteins such as Igs and the TCRs.
These are formed by the variable regions on light and heavy
chains. These variations are individually called idiotopes and
collectively known as idiotypes.

49. Affinity and avidity of antibodies
Affinity
The tightness with which the antigen binding site attaches to an
antigen determinant (epitope)
Avidity
The tightness of binding when several antigen binding sites
attach to several antigenic determinants

50. Physical forces holding antibodies and antigen together

51. • Plasma cells make over a billion different types of
antibodies
• Each cell, however, only contains 100,000 genes that code
for these polypeptides
• To code for this many antibodies, somatic recombination
takes place
– Gene segments are shuffled and combined in different
ways by each B cell as it becomes immunocompetent
– Information of the newly assembled genes is expressed
as B cell receptors and as antibodies
Mechanisms of Antibody Diversity

52. • Random mixing of gene segments makes unique antibody
genes that:
– Code for H and L chains
– Account for part of the variability in antibodies
• V gene segments, called hypervariable regions, mutate and
increase antibody variation
• Plasma cells can switch H chains, making two or more
classes with the same V region
Antibody Diversity

53. • Antibodies themselves do not destroy antigen; they
inactivate and tag it for destruction
• All antibodies form an antigen-antibody (immune) complex
• Defensive mechanisms used by antibodies are
neutralization, agglutination, precipitation, and complement
fixation
Antibody Targets

54. • Complement fixation is the main mechanism used against
cellular antigens
• Antibodies bound to cells change shape and expose
complement binding sites
• This triggers complement fixation and cell lysis
• Complement activation:
– Enhances the inflammatory response
– Uses a positive feedback cycle to promote phagocytosis
– Enlists more and more defensive elements
Complement Fixation and Activation

55. • Neutralization – antibodies bind to and block specific sites
on viruses or exotoxins, thus preventing these antigens
from binding to receptors on tissue cells.
• Agglutination – antibodies bind the same determinant on
more than one antigen, makes antigen-antibody complexes
that are cross-linked into large lattices.
• Cell-bound antigens are cross-linked, causing clumping
(agglutination).
• Precipitation – soluble molecules are cross-linked into large
insoluble complexes
Other Mechanisms of Antibody Action

56. Mechanisms of Antibody Action