antigens and immunogens

1. Antigens & Immunogens

2. Antigen
 An antigen is a molecule that stimulates an immune
response.
 The word originated from the notion that they can
stimulate antibody generation

3. Immunogen
* A foreign substance, when introduced into
host body, stimulate formation of specific
antibodies or sensitized lymphocytes
Antigen
* Antigens have the ability to combine
specifically with antibodies produced or
sensitized T-lymphocytes induced

4.  Immunogenicity: ability to induce immune
response in the body (both humoral and/or cell
mediated)
 Antigenicity: ability to combine specifically with
the final products of specific immune response
(antibodies and/or T cell-surface receptors)
 All immunogens are antiges, but all the antigens
are not immunogens

5. Epitope or antigenic determinant
Small area present on the antigen comprising of few (four to
five) amino acids or monosaccharide residues, that is
capable of sensitizing T and B cells & reacting with specific
site of T cell receptor or an antibody
Small part of an antigen that interacts with an antibody
• Any given antigen may have several epitopes.
• Each epitope is recognized by a different antibody or
TCR

6. Types of epitope

7. Antigenic epitopes

8. Haptens
Low molecular weight molecules that lack immunogenicity
(cannot induce immune response) but retain antigenicity or
immunological reactivity (i.e. can bind to their specific
antibody or T cell receptor)
Haptens can become immunogenic when combined with a
larger protein molecule called ‘carrier’
The hapten-carrier complex is capable of inducing immune
response in the body
- Examples :
simple chemicals and drugs: penicillin, sulphonamide,
aspirin, cosmetic, tranquillizers, neomycin skin ointment

9. Animals immunized with such a hapten-
carrier conjugate produce antibodies
specific for-
oEpitopes of hapten
oUnaltered epitopes on the carrier protein
&
oNew epitopes formed by combined parts
of both the hapten and carrier

10. Factors influencing Immunogenicty

14. 5- Method of administration:
a- Antigen dose:
Appropriate dose optimum antigenicty
Low dose low- zone tolerance
High dose high-zone tolerance
Repeated Number of doses of antigens
Multiple antigens
Effect of prior administration of antibody
b- Adjuvant:
Substance when injected with an antigen
enhance immunogenicty

15. 6. Susceptibility of antigen to tissue enzymes
Phagocytosis & intracellular enzymes break the
antigen into immunogenic fragments. D amino acids
are not antigenic (not metabolised) while
polypeptides consisting L amino acids are
antigenic.
7. Genetic factor
Different individuals of a given species show different
types of immune responses towards the same antigen.
Responders, Slow responders & Non-responders
MHC

16. Heterophile antigens

17. Diagnostic application-

19. Y
T dependent Ag - T cell help to B cells
B
Signal 1
antigen & antigen
receptor
Th
1. T cell antigen receptor
2. Co-receptor (CD4)
3.CD40 Ligand
Th
Signal 2 - T cell help IL-2 and other cytokines

20. T-independent antigens
 polysaccharides and proteins with many identical
polypeptides
 Many bacterial capsules & bacterial flagellar proteins
 This stimulates the B cell to generate antibody-secreting
plasma cells without the help of IL-2.
 While this response is an important defense against many
bacteria, it generates a weaker response than T-dependent
antigens and generates no memory cells.

21. Thymus independent antigens…
Principal example:
LPS

22. Principal example(s):
Bacterial capsules
Bacterial flagella
Mechanism:
extensive cross-linking of mIg

23. T
Dependent
Antigens
TI-1
Antigens
TI-2
Antigens
Induce responses in athymics No Yes Yes
Polyclonally activate B cells No Yes No
Require repeating epitopes No No Yes
Affinity maturation Yes No No
Isotype switching Yes No Limited
Immunological memory Yes No No
T Dependent & Independent Antigens

24. Superantigens

25. Superantigens
Directly bridge non-specifically between
major histocompatibility complex (MHC)-II
of APCs and T cells.
Non-specific activation of T cells leads to
massive release of cytokines

26. Conventional Ag
Superantigens

27. Bacterial superantigen
Staphylococcal toxin-
 Toxic shock syndrome toxin-1(TSST-1); Exfoliative
toxin;Enterotoxins
Streptococcal toxin- Streptococcal pyrogenic exotoxin (SPE)-
A and C
Mycoplasma arthritidis mitogen-I
Yersinia enterocolitica
Yersinia pseudotuberculosis
Viral superantigen
Epstein-Barr virus associated superantigen
Cytomegalovirus associated superantigen
Rabies nucleocapsid
HIV encoded superantigen (nef- negative regulatory factor)
Fungal superantigen
Malassezia furfur

28. Disease associated with superantigens
 Conditions associated with staphylococcal
toxins are as follows-
o Toxic shock syndrome
o Food poisoning
o Scalded skin syndrome
o Rare conditions such as- Atopic dermatitis,
Kawasaki syndrome, psoriasis, acute
disseminated encephalomyelitis.

29. Cross reacting antigen
 Ags sharing identical/similar epitope
 Sometimes, Ab can “cross-react” with unrelated Ag….
 Often seen with polysaccharide Ag’s
 e.g. ABO Blood groups – glycoproteins
 -persons lacking one or both of the blood (AB)
Ag’s will have serum Ab’s vs.the missing Ag’s
 -these Ab’s produced from cross-reactive MO
Ag’s!!
 -provides basis for blood typing tests

30. -
The outer layer of RBCs membrane contain certain
antigens which are chemically glycoproteins &
called agglutinogens.
-According to the presence of these antigens ,
human blood can be classified into many groups.

31. ABO system
-This system includes 2 types of antigens A and B agglutinogens.
-According to the presence of A and B antigens there are 4 groups of
human blood :
1- Group A (about 41% ) : when only type A antigen is present.
2- Group B (about 9 % ) : when only type B antigen is present.
3- Group AB (about 3 % ): when both types A and B antigens are present.
4- Group O (about 47 % ): when both types A and B antigens are absent.

32. A: adds N-acetylgalactosamine to the terminal sugar of the H
antigen
B: adds D-galactose to the terminal sugar of the H antigen

35. Rh system (Rh factor)
-This is system of agglutinogens normally present in the red
cells of Rhesus monkey (hence the name Rh)
-There are 6 varieties known as(dominant C , D and E)
(antigens and recessive c , d and e ) antigens.
-These antigens were also found in the human red cells.
-Antigen D has the strongest antigenic effect So :
If D antigen is present → Rh +ve.
If D antigen is absent → Rh -ve.

36. Significances of blood grouping:
1- Blood transfusion.
2-Marriage ( Rh incompatibility).
3-Disputed paternity.
4- Detection of criminals(Medico-legal).

37. Determination of ABO system
-Mix one drop of blood with 1 ml of isotonic saline in
a test tube.
-Put 2 separate drops of the diluted blood on a glass
slide.
-Add one drop of anti-A serum (blue coloured) to
one blood drop and one drop of anti-B (yellow
coloured) to the other drop.
-Mix the blood with anti-A and anti-B sera gently
using the blunt end of two separate pins.

38. -After 2 minutes examine for agglutination :
a- If agglutination occurs with anti-A serum only , the
blood group is type A.
b- If agglutination occurs with anti-B serum only , the
blood group is type B.
c- If agglutination occurs with both anti-A and anti-B sera ,
the blood group is type AB.
d- If no agglutination occurs with either anti-A or anti-B ,
the blood group is type O.

39. Determination of Rh group
-Put 2 drops of diluted blood on a glass slide.
-Add one drop of anti-D serum to the blood and mix
gently with the blunt end of a pin.
-Examine for agglutination :
a-If agglutination occurs , the blood group is
Rh +ve.
b-If no agglutination occurs , the blood group
is Rh –ve.

41. Mitogen