Antigens are substances that stimulate the immune system to produce antibodies. The smallest part of an antigen that induces an immune response is called an epitope. Antigens can be proteins, polysaccharides, lipids, or nucleic acids. Antigens must be foreign to induce an immune response, as the body develops tolerance to self antigens. Antibody molecules bind specifically to antigens. Antibodies belong to five classes - IgG, IgA, IgM, IgD, and IgE - which have different structures and functions such as neutralizing toxins, activating complement pathways, and aiding phagocytosis. Abnormal immunoglobulins can indicate diseases like multiple myeloma or cryoglobulinemia.

1. Antigen and antibody
Dr. Himanshu Khatri
Email: himanshubkhatri@yahoo.co.in

2. Definition
• An antigen is any substance which, when
introduced into the body, usually stimulates
the production of an antibody
• It reacts with it specifically, and in an
observable manner

4. Attributes of antigenicity
• Immunogenicity: induction of immune
response
• Immunological reactivity: specific reaction
with antibodies or sensitised cells

5. Antigen, hapten and adjuvant
• Antigen exhibits both immunogenicity and
immunological reactivity
• Hapten exhibits only immunological reactivity
• Haptens can exhibit immunogenicity on
combining with larger carrier molecule
• Adjuvants are substance which, when mixed
with antigens, increase their immunogenicity

8. Antigenic determinant or epitope
• Smallest unit of antigenicity
• Small area of antigen consisting of 4-5 amino
acids or monosaccharide residues
• Having a specific chemical structure, electrical
charge and spatial configuration
• Capable of immunogenicity

11. Cross reactivity
• Stereochemical similarities is responsible for
most cases of cross-reactivity
• The presence of same or similar epitopes on
different antigens is responsible for some of
the antigenic cross reactions

12. Paratope
• The area on the antibody molecule
corresponding to the epitope

13. Determinants of antigenicity
• Size
• Chemical nature
• Susceptibility to tissue antigens
• Foreignness

14. Size
• Antigens with larger molecular size are more
immunogenic
• Smaller molecules are less immunogenic, and
may be related to hypersensitivity

15. Chemical nature
• Proteins and polysaccharides are more
immunogenic
• Lipids and nucleic acids are less immunogenic
• Proteins are more immunogenic than
polysaccharides

16. Susceptibility to tissue enzymes
• Only substances which can be degraded by
enzymes to immunogenic fragments are
capable of being antigens
• Non- degradable substances like polystyrene
latex are non-immunogenic

17. Foreignness
• Only antigens which do not belong to the self
are immunogenic
• This is because the body develops tolerance to
self antigens during development of immune
system
• Antigens from other individuals of the same
species are less than those from other species
• Antigens from related species are less
immunogenic than those from distant species

18. Antigen specificity
• Species specificity
• Isospecificity
• Autospecificity
• Organ specificity
• Heterogenetic (heterophile) specificity

19. Species specificity
• Tissues of all individual in a species contain
species specific antigen

20. Isospecificity
• Some antigens are found in some, but not all,
members of a species
• For example, the blood group antigens in
humans are of two types A and B. People
having antigens A, B, both, and none on RBCs
are designated blood groups A, B, AB, and O
respectively

21. Autospecificity
• Some antigens are specific to an individual
• The immune system does not respond to that
antigen, because during development of
immune apparatus, the immune system
develops tolerance to that antigen

22. Organ specificity
• Some antigens are specific for organs, and
may be present in different species
• The brain of sheep and humans share a
common antigen. This is why, when people
are vaccinated against rabies with vaccine
prepared from sheep brain, the immune
response attacks the human brain, and causes
neuroparalytic complications

23. Heterophile specificity
• Same or similar antigens may be shared across
species, classes, and kingdoms
• For example, Rickettsia and Proteus share
some same antigens. So, the antigens from
Proteus species can be used to detect
antibody response against Rickettsia species.
This is the basis of Weil-Felix test

24. Biological classes of antigens
• Antibodies are secreted by plasma cells. Plasma
cells are differentiated B cells
• For the full expression of function, some
cooperation from T-cells is necessary, and the
antigens are first recognized by the T-cells (T-
dependant or TD antigens)
• However, some antigens can be directly
recognized by B-cells, bypassing the recognition
by T-cells (T-independent or TI antigens)

25. • TI antigens
1. Usually polysaccharide
or lipopolysaccharide
(LPS)
2. Too little amount is non-
immunogenic and too
much results in
tolerance
3. Immunological memory
is absent
• TD antigens
1. Usually proteins
2. Immune response is not
dose dependant
3. Immunological memory
is present

26. Antibodies
• Antibodies are proteins in the serum which
react with an antigen specifically, and in an
observable manner
• They are a part of a globulin fraction of the
serum

27. Antibodies - Immunoglobulins
• Immunoglobulins are proteins of animal origin
which have a known antibody activity or
chemically related proteins without an
antibody activity
• All immunoglobulins do not have antibody
activity, but all antibodies are structurally
immunoglobulins
• Immunoglobulins is a structural concept,
whereas antibody is a functional concept

28. Structure of immunoglobulins
• Each immunoglobulin (Ig)
molecule consists of two
heavy (H) and two light
(L) chains
• Each L chain is linked to
each H chain with the
help of a disulphide bond
• Two H chains are linked to
each other with the help
of a disulphide bond

29. H chains
• There are five classes of Ig molecules
depending on the type of H chain in them
Immunoglobulin class H chain
IgG γ ( gamma)
IgA α (alpha)
IgM μ (mu)
IgD δ (delta)
IgE ε (epsilon)

30. L chains
• L chains occur in two varieties, kappa (κ) and
lambda (λ)
• A molecule of Ig may have two kappa or two
lambda chains, but never one of each

32. Constant and Variable regions
• The H and L chains of each Ig molecule
consists of constant regions (which are
structurally same in all Igs belonging to a class
(γ, α, μ, δ, or ε) and variety (κ or λ)
• The H and L chains also contain variable
regions, which differ in their structure across
IgGs
• The variable region is found towards the
antigen binding terminus of the IgG

33. Hypervariable regions
• The variable regions contain a relative
invariable region and some highly variable
regions (or hot spots)
• The sites on the highly variable regions, which
make contact with the epitope, are called
complementarity determining regions or CDRs

34. Fc and Fab fractions
• The enzyme papain acts at the hinge region of
the Ig molecule, and divides it into one Fc and
two Fab fragments
• The Fab fragment (ab for antigen binding)
contains the antigen binding site
• The Fc fragment (c for crystallisable) contains
the site for binding of the complement and
macrophages

35. Immunoglobulin classes

36. IgG
• Most abundant of all Igs (80%)
• Monomer
• Divided equally in intra- and extra-vascular space
• Only Ig to cross placenta from the mother to
fetus
• Participates in most immunological reactions like
complement fixation, precipitation, and
neutralization of toxins and viruses
• Has a very long half-life, and persists for long
periods in the body

37. IgA
• Occurs as serum IgA and secretory IgA
• Secretory IgA (SIgA) is a dimer, where the two IgA
molecules are joined by a J chain (J for joining)
• SIgA is found at mucosal surfaces and in
secretions
• SIgA also contains a secretory piece, which
protects it from degradation by enzymes
• It does not fix the complement, but can activate
the alternate complement pathway

38. IgM
• Pentamer
• Five molecules of Ig are joined by a J chain
• Mostly (80% of IgM) intravascular is distribution, IgM
deficiency is associated with septicemia
• Earliest Ig to be synthesized during immune response
• Very short half life, hence their demonstration indicates
recent infection
• Does not cross the placenta, and hence used for detection
of infection in neonates
• Very strong agglutination and lysis action
• Monomeric IgM is the major Ig on surface of B lymphocytes
for antigen recognition

39. IgD
• Mostly intravascular
• Present on the surface of B lymphocytes,
along with monomeric IgM, for antigen
recognition

40. IgE
• Mostly extravascular in origin
• Present on the surface of mast cells and
basophils
• Levels increase in atopic conditions like
asthma, hay fever, etc.,
• Levels also increase during parasitic infections

41. Functions of Igs
• Igs are produced by the plasma cells
(differentiated B cells), and perform the following
functions:
1. They prevent attachment of microorganisms to
mucosal surfaces (IgA)
2. They neutralize toxins of microorganisms
3. They facilitate binding of complement, and
activation of its pathway
4. They opsonize the microorganisms for more
effective phagocytosis

42. Abnormal immunoglobulins
• They are seen in:
1. Multiple myeloma
2. Waldenstrom’s macroglobulinemia
3. Cryoglobulinemia

43. Multiple myeloma
• Cancerous plasma cells produce large
amounts of Igs
• Plasma cells producing IgG, IgA, IgD or IgE may
be affected

44. Waldenstrom’s macroglobulinemia
• Cancerous plasma cells produce large
amounts of IgM

45. Bence Jones proteins
• Are light chains
• Are either κ or λ in any single patient
• Can be identified in urine by its typical
property of coagulation at 50oC, but
redissolving at 70oC
• Are usually found in Multiple Myeloma, but
may be also found in Waldenstrom’s
macroglobulinemia

46. Cryoglobulinemia
• Cryoglobulins are immunoglobulins or their
fragments; or proteins of the complement
system
• They precipitate on cooling but redissolve on
warming
• They are not only found in multiple myeloma
and Waldenstrom’s macroglobulinemia, but
also in autoimmune conditions like systemic
lupus erythematosus

47. Self study
• Immunoglobulin specificities
• Class switching in immunoglobulins
• Antibody diversity

48. THANK YOU