Antigens are substances that induce an immune response when introduced to a host. They have two key characteristics - immunogenicity, which is the ability to stimulate an immune response, and antigenicity, which is the ability to react with antibodies or sensitized T cells. Antigens can be classified as foreign, auto, iso, or hetero antigens depending on their source. Important antigen types include haptens, epitopes, and superantigens. The immune system responds to antigens through antibodies and T cell activation.

1. ANTIGENS
an antigen (the generator) when
introduced into a host induces the
formation of specific antibodies and
sensitize T lymphocytes that are
reactive against that specific antigen

2. Definitions
TWO IMPORTANT CHARACTERISTICS 0F AN ANTIGEN
1) IMMUNOGENICITY: The ability of a substance to
stimulate a specific immune response
2) ANTIGENICITY : The ability of a substance to react
specifically with antibodies/ sensitized T cells
• HAPTEN: are small molecules that cannot induce an
immune response by themselves unless complexed
with a large carrier molecule like protein/
polysaccharide
• EPITOPE or Antigenic Determinant: that portion
of an antigen that combines with the product of a
specific Immune response
• ANTIBODY (Ab): a specific protein produced in
response to an immunogen

3. Factors Influencing
Immunogenicity
• Foreignness
• Size- Macromolcule
• Chemical Composition
Complexity of structure
• Physical Form
Particulate / Soluble
Denatured / Native
Biodegradability
Ag processing by Ag Presenting Cells (APC)

4. CLASSIFICATION OF ANTIGEN
• FOREIGN (OUTSIDE THE HOST)
MICROBE COMPONENTS/ PRODUCTS,
DRUGS
ENVIRONMENTAL ANTIGENS
• AUTOANTIGENS
THYROGLOBULIN, NUCLEIC ACID,
CORNEA COMPONENTS
• ISOANTIGENS
HISTCOMPATIBILITY TISSUE ANTIGENS
BLOOD GROUP ANTIGENS
• HETROANTIGENS
HETROPHILE ANTIGENS
CROSS REACTING MICROBIAL ANTIGENS
eg. Streptococcal cell wall antigen cross react
with human cardiac and glomerular tissue

5. ANTIGEN EPITOPES ON BACTERIAL CELL
EPITOPES /
ANTIGENIC DETERMINANTS:
Chemical groups on
the antigenic molecule
against which antibodies
are produced.
Eg: bacterial cell–
have 100 or more epitopes

7. Chemical Nature of
Immunogens
• Proteins– pure or glycoproteins or lipoproteins
• Polysaccharides– pure or lipopolysac
• Nucleic Acids- poor immunogens or
become immunogen when single stranded
or complexed with proteins
• Lipids– generally non immunogens
Some glycolipids and phosopholipids
can be immunogenic for T cells and
illicit a cell mediated immune response

8. Types of Antigens
1)T-independent Antigens
(directly stimulates B cells to produce antibodies without
help of T cells)
• POLYSACCHARIDES
• Examples
Pneumococcal
polysaccharide
 lipopolysaccharide
Flagella

9. Types of Antigens
2) T-dependent antigens
(do not directly stimulate Ab formation without
the help of T cells)
• PROTEINS
• Examples
Microbial
proteins
Non-self or
Altered-self
proteins

10. ANTIGENS
HEPTENS--- small molecules that cannot
induce an immune response unless complexed with
a large carrier molecule like protein/ polysac
then the hepten alone can react specifically with the
antibodies or immune cells produced in response to
the hepten-carrier complex.
Two types:
simple- one antigenic
determinant
complex- many antigenic
determinants
Native determinants
Haptenic determinants
Are immunogenic molecules to which heptans become attached
Hapten-carrier conjugates:

11. Hapten-carrier conjugates
Definition:
Are immunogenic molecules to which
heptans become attached
Native determinants
Haptenic determinants
• Structure
native
determinants
haptenic
determinants
(created by Heptans)

12. SUPER ANTIGENS
• Definition:
T-dependent antigens
cause massive activation
of large fraction (25%) of
T cells (polyclonal
response) ---uncontrolled
release of cytokines
• Examples
 Staphylococcal enterotoxins
 Staphylococcal toxic shock
toxin
 Staphylococcal exfoliating
toxin
 Streptococcal pyrogenic
exotoxins

13. IMMUNOGLOBULINS:
Structure and Function

14. Immunoglobulins:Structure and
Function
• DEFINITION: Glycoprotein molecules that are
produced by plasma cells in response to an
immunogen and which function as antibodies
• FUNCTIONS:
• Recognizing and binding to foreign substances
and facilitating their removal
• Binding to various cells– phagocytic, lymphocytes
mast cells, basophils, placental trophoblast
• Increase phagocytosis
• Neutralizing toxins and viruses
• Activating complement

15. Immunoglobulin Structure
• Identical : 2-
Heavy(H) & 2
Light Chains-L
• Disulfide
bonds
Inter-chain
Intra-chain
CH1
VL
CL
VH
CH2 CH3
Hinge Region
Carbohydrate
Disulfide bond

16. Immunoglobulin Structure: on
basis of their A. acid sequence.
• DOMAINS
• Variable (V)
• Constant (C)
Regions
 VL & CL
 VH & CH
• Hinge Region
• CL 110 A.Acid
• VL 110
• VH 11O
• CH 33O—44O
CH1
VL
CL
VH
CH2 CH3
Hinge Region
Carbohydrate
Disulfide bond

17. Immunoglobulin Fragments:
Structure/Function Relationships
• Fab
Ag binding
Valence = 1
Specificty
determined by VH
and VL
Papain
Fc
Fab
• Fc
CH2 __CH3
Effector functions

18. Immunoglobulin Fragments:
Structure/Function
Relationships
Ag Binding
Complement Binding Site
Placental Transfer
Binding to Fc
Receptors- M*,
Polymorps

19. Human Immunoglobulin Classes
• ON BASIS OF DIFFERENCE IN A.ACID
SEQUENCE IN CONSTANT REGION OF
HEAVY CHAINS:
• IgG - Gamma () heavy chains
• IgM - Mu () heavy chains
• IgA - Alpha () heavy chains
• IgD - Delta () heavy chains
• IgE - Epsilon () heavy chains

20. Human Immunoglobulin
Subclasses
• IgG Subclasses
IgG1 - Gamma 1 (1) heavy chains
IgG2 - Gamma 2 (2) heavy chains
IgG3 - Gamma 3 (3) heavy chains
IgG4 - Gamma 4 (4) heavy chains
• IgA subclasses
IgA1 - Alpha 1 (1) heavy chains
IgA2 - Alpha 2 (2) heavy chains

21. Human Immunoglobulin
Light Chain Types
• Kappa ()
• Lambda ()
• Subtypes Lambda light chain:
Lambda 1 (1)
Lambda 2 (2)
Lambda 3 (3)
Lambda 4 (4)

22. IgG
• Structure
Monomer (7S)
Subclasses differ in number of disulfide bonds
and length of hinge region
IgG1, IgG2 and IgG4 IgG3

23. IgG
• Properties
• Most versatile immunoglobulin
Major serum Ig- 75% of serum Ig
Major Ig in extravascular spaces
Placental transfer – Does not require Ag
binding ( IgG2)
Fixes complement ( IgG4)
Binds to Fc receptors ( IgG2, IgG4)
 Phagocytes - opsonization
 K cells - ADCC

24. IgM
• Structure
Pentamer
mu chains
Extra domain (CH4)
J chain
C4
J Chain

25. IgM
Properties
3rd highest serum Ig
First Ig made by
fetus and B cells
Fixes complement
Agglutinating
antibody
Binds to Fc
receptors

26. IgA
• Structure
Serum - monomer
Secretions (sIgA)
 Dimer –
 J chain
 Secretory
component-
recruited from
serous epith.
J ChainSecretory Piece

27. Origin of Secretory Component of
sIgA

28. IgA
Properties
2nd highest serum Ig
1ST
line of defense in mucous membrane
pathogen
Major secretory Ig (Mucosal or Local Immunity)
 Tears, saliva, gastric and pulmonary secretions
Does not fix complement (unless aggregated)
Binds to Fc receptors on some cells
Neutralizes viruses and toxins

29. IgD
• Structure
Monomer
Tail piece
Tail Piece

30. IgD
Properties
4th highest serum Ig
B cell surface Ig
Does not bind complement
Increased levels in AIDS

31. IgE
• Structure
Monomer
Extra domain (CH4)
C4

32. IgE
Properties
Least common serum Ig
 Binds to basophils and mast cells (does not require
Ag binding)
Allergic reactions
Parasitic infections (Helminths)
 Binds to Fc receptor on Eosinophils
 Activation of macrophages--- ADCC
Does not fix complement

34. Antibodies eliminate foreign agents in 3 ways

35. Characteristics of Various Classes of
Immunoglobins

36. Properties of Antibiotics

37. MHC COMPLEX

38. MAJOR HISTOCOMPATIBLE -MHC
COMPLEX
• MHC complex defines uniqueness of the individual
• Part of the genome encoding for MHC antigens– clustered on a
small segment of chromosome 6– CONSTITUTE HLA
COMPLEX
• At least 20 different genes encode for MHC PROTEINS
• Variety of MHC proteins exist in humans
• Identical twins have same MHC molecules
• All family members have a mixture of similar and different MHC
molecules
• Functions:
• The principle function is to bind peptide fragments
of foreign proteins and present it to antigen specific
T cell.
• Immune recognition
• Interaction between B and T and other cells
• Rejection of allograft

39. MHC COMPLEX
• MHC 1 ANTIGENS MHC II ANTIGENS
1.Present on the surface Present on the B cell,
of all nucleated cells Macrophages and
of the body, platelets activated T cells
2. Bind Ag/ peptide Bind Ag/ peptides
endogenously produce exogenously produced
eg. viral proteins then processed within
cell after phagocytosis
3. Present Ag to CD8 - Present Ag to CD4 cells-
TC cells TH Cells
CLASS1 AND 11 GENES ENCODE CELL SURFACE GLYCOPROTEINS
AND CLASS III GENES FOR COMPONENTS OF COMPLEMENT
SYSTEM

42. CYTOKINES

43. CYTOKINES
• Short acting soluble mediators released during
interaction between lymphocytes, monocytes,
macrophages, inflammatory cells and endothelial cells
• Concerned with induction and regulation of immune
response
• Hormone like peptides or glycopeptides with wide
spectrum of effects
• PROPERTIES
• Individual cytokines produced by different cell types eg.
IL-I,TNF -- PRODUCED BY VIRTUALLY ALL CELLS
• Cytokines have PLEOTROPIC effect– act on many cell
types—IL2 ---T cells, B, NK CELLS
• Cytokines produce their effect by combining with high
affinity receptors on the target cell eg:IL2 activate T cells
by binding to IL2 receptor
• They can act on the same cell that produces it eg IL 2
produced by activated T cell promote T cell growth

45. Important Cytokines