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ANTIGEN


1. Definition of ANTIGEN

An antigen is any substance which, when introduced into the body, induces an immune response and specifically reacts with antibodies or sensitized T-cells.
👉 Substances may be foreign or self, but immunogenic antigens are usually foreign molecules.
2. Immunogen vs Antigen

Immunogen
Substance that induces immune response
Antigen
Substance that reacts with immune products
Hapten
Antigenic but not immunogenic alone
👉 All immunogens are antigens, but all antigens are not immunogens.


3. Chemical Nature of Antigens
Antigens may be:
a) Proteins (Most potent)
Enzymes
Toxins
Structural proteins

b) Polysaccharides
Bacterial capsules
Cell wall components
c) Glycoproteins
Viral envelope proteins
d) Lipids & Nucleic acids
Weakly antigenic
Become immunogenic when combined with proteins


4. Properties of Antigens
An ideal antigen shows:
Foreignness
High molecular weight (>10,000 Da)
Chemical complexity
Stability
Specificity
Degradability (processing by APCs)


5. Types of Antigens

a) Exogenous Antigens
Enter from outside
Examples: bacteria, viruses, pollen
Processed by APCs → presented on MHC-II

b) Endogenous Antigens

Produced inside cells
Examples: viral proteins
Presented on MHC-I

c) Autoantigens

Self antigens
Cause autoimmune diseases
Example: DNA in SLE
d) Alloantigens
Present in members of same species
Example: Blood group antigens

e) Heteroantigens (Xenoantigens)

From different species
Example: animal serum proteins

6. Complete and Incomplete Antigens

Complete Antigen
Immunogenic + antigenic
Example: bacterial proteins
Incomplete Antigen (Hapten)
Antigenic but not immunogenic alone
Needs carrier protein
Example: penicillin

7. Haptens
Low molecular weight substances
Become immunogenic when attached to carrier
Used in vaccine development
Example:
Penicillin + serum protein → antigen


8. Antigenic Determinant (Epitope)

An epitope is the specific site on antigen that binds with antibody or T-cell receptor.
Types:
B-cell epitope – surface, conformational
T-cell epitope – linear peptide, MHC-restricted
9. Valency of Antigen
Valency = number of epitopes per antigen molecule
High valency → stronger immune response


10. Antigen–Antibody Interaction

Key features:
Specific
Reversible
Non-covalent bonds
Types of bonds:
Hydrogen bonds
Ionic bonds
Van der Waals forces
Hydrophobic interactions


11. Factors Affecting Antigenicity
Molecular size
Structural complexity
Foreignness
Dose of antigen
Route of administration
Genetic makeup of host
Presence of adjuvants


12. Adjuvants

Substances that enhance antigen immunogenicity.
Examples:
Alum
Freund’s adjuvant
Functions:
Prolong antigen presence
Stimulate APCs


13. Superantigens


Activate large number of T-cells
Bind outside antigen-binding groove
Cause massive cytokine release
Example:
Staphylococcal enterotoxins


14. Antigens in Vaccines


Induce protective immunity
Types used:
Live attenuated
Killed
Subunit
Recombinant antigens


15. Biological Significance of Antigens
Basis of immunity
Vaccine development
Disease diagnosis
Transplant rejection
Autoimmune disorders


16. Conclusion
Antigens are essential components of the immune system, responsible for immune recognition and response. Understanding their structure, types, and properties is crucial for immunology, vaccination, and clinical diagnosis.



50 MCQs ON ANTIGENS


1. An antigen is best defined as a substance that
A. Is always a protein
B. Induces fever
C. Reacts specifically with antibody or T-cell receptor
D. Is always foreign
Answer: C

2. A substance that can induce immune response is called
A. Antibody
B. Hapten
C. Immunogen
D. Adjuvant
Answer: C


3. All immunogens are
A. Haptens
B. Antibodies
C. Antigens
D. Adjuvants
Answer: C

4. Which of the following is the most potent antigen?
A. Lipids
B. Proteins
C. Nucleic acids
D. Polysaccharides
Answer: B


5. The minimum molecular weight required for good antigenicity is
A. 1,000 Da
B. 5,000 Da
C. 10,000 Da
D. 100 Da
Answer: C


6. Foreignness is important for
A. Antibody degradation
B. Antigenicity
C. Tolerance
D. Autoimmunity
Answer: B


7. Which antigen is produced inside the host cell?
A. Exogenous antigen
B. Autoantigen
C. Endogenous antigen
D. Heteroantigen
Answer: C


8. Exogenous antigens are presented by
A. MHC-I
B. MHC-II
C. CD8 cells
D. NK cells
Answer: B


9. Endogenous antigens are presented by
A. MHC-II
B. Antibody
C. MHC-I
D. APC only
Answer: C


10. Autoantigens are responsible for
A. Allergy
B. Vaccination
C. Autoimmune diseases
D. Transplant rejection
Answer: C


11. Heteroantigens are obtained from
A. Same individual
B. Same species
C. Different species
D. Pathogens only
Answer: C


12. An antigen that is antigenic but not immunogenic alone is called
A. Immunogen
B. Adjuvant
C. Hapten
D. Epitope
Answer: C

13. Haptens require ______ to become immunogenic
A. Antibody
B. Carrier protein
C. MHC molecule
D. Complement
Answer: B


14. Penicillin is an example of
A. Complete antigen
B. Autoantigen
C. Hapten
D. Superantigen
Answer: C

15. Antigenic determinant is also called
A. Paratope
B. Epitope
C. Idiotype
D. Isotype
Answer: B


16. The binding site of antibody is called
A. Epitope
B. Fc region
C. Paratope
D. Constant region
Answer: C
17. B-cell epitopes are usually
A. Linear
B. Hidden
C. Surface-exposed
D. Intracellular
Answer: C
18. T-cell epitopes are
A. Conformational
B. Lipid based
C. Linear peptides
D. Polysaccharides
Answer: C
19. Valency of antigen refers to
A. Charge on antigen
B. Size of antigen
C. Number of epitopes
D. Molecular weight
Answer: C
20. Which bond is strongest in antigen-antibody interaction?
A. Covalent bond
B. Ionic bond
C. Hydrogen bond
D. Van der Waals force
Answer: A
21. Antigen-antibody reactions are
A. Irreversible
B. Covalent
C. Highly specific
D. Enzymatic
Answer: C
22. Which of the following is NOT a factor affecting antigenicity?
A. Molecular size
B. Route of administration
C. Host genetics
D. Blood pressure
Answer: D
23. High dose of antigen may cause
A. Strong response
B. Tolerance
C. Allergy
D. Fever
Answer: B
24. Adjuvants enhance immune response by
A. Destroying antigens
B. Neutralizing antibodies
C. Prolonging antigen presence
D. Removing epitopes
Answer: C
25. Commonly used adjuvant in vaccines
A. Alum
B. Penicillin
C. Histamine
D. Interferon
Answer: A
26. Superantigens activate
A. One T-cell clone
B. Few B-cells
C. Large number of T-cells
D. Only macrophages
Answer: C
27. Superantigens bind to
A. Antibody only
B. MHC outside peptide groove
C. Epitope only
D. Fc region
Answer: B
28. Staphylococcal toxins are
A. Haptens
B. Adjuvants
C. Superantigens
D. Autoantigens
Answer: C
29. Blood group antigens are examples of
A. Autoantigens
B. Alloantigens
C. Heteroantigens
D. Hapten
Answer: B
30. An antigen with many epitopes shows
A. Low immunogenicity
B. No reaction
C. High immunogenicity
D. Tolerance
Answer: C
31. Lipids become antigenic when
A. Heated
B. Combined with proteins
C. In high dose
D. Injected orally
Answer: B
32. Polysaccharide antigens mainly induce
A. Cell mediated immunity
B. IgE response
C. T-cell dependent response
D. T-cell independent response
Answer: D
33. Which antigen is responsible for transplant rejection?
A. Autoantigen
B. Alloantigen
C. Hapten
D. Superantigen
Answer: B
34. Antigens used in vaccines are mainly
A. Self antigens
B. Protective antigens
C. Autoantigens
D. Carrier proteins
Answer: B
35. Epitope-paratope interaction is
A. Enzymatic
B. Non-specific
C. Reversible
D. Covalent
Answer: C
36. Antigens that induce allergy are called
A. Toxins
B. Allergens
C. Haptens
D. Autoantigens
Answer: B
37. Which cell presents antigen to T-cells?
A. RBC
B. Platelets
C. Antigen-presenting cells
D. Mast cells
Answer: C
38. MHC-II molecules are found on
A. All body cells
B. RBCs
C. APCs
D. Neurons
Answer: C
39. The immune response against self antigens is prevented by
A. Immunization
B. Memory cells
C. Self tolerance
D. Adjuvants
Answer: C
40. Chemical complexity increases
A. Toxicity
B. Antigenicity
C. Solubility
D. Diffusion
Answer: B
41. Antigens are essential for
A. Metabolism
B. Respiration
C. Immune recognition
D. Cell division
Answer: C
42. Antigens reacting with IgE cause
A. Autoimmunity
B. Hypersensitivity
C. Tolerance
D. Immunodeficiency
Answer: B
43. An example of endogenous antigen is
A. Bacterial toxin
B. Pollen
C. Viral protein inside cell
D. Snake venom
Answer: C
44. Antigens that cross species barrier are
A. Autoantigens
B. Alloantigens
C. Xenoantigens
D. Neoantigens
Answer: C
45. Neoantigens are formed by
A. Mutation
B. Vaccination
C. Aging
D. Digestion
Answer: A
46. Antigen processing occurs in
A. Ribosome
B. Lysosome
C. Nucleus
D. Golgi body
Answer: B
47. Which immune response is antigen-specific?
A. Innate
B. Adaptive
C. Inflammatory
D. Physical barrier
Answer: B
48. Antigen recognition in T-cells is
A. Direct
B. MHC-restricted
C. Antibody mediated
D. Complement dependent
Answer: B
49. Antigens involved in cancer immunity are
A. Tumor antigens
B. Autoantigens
C. Haptens
D. Allergens
Answer: A
50. Antigen-antibody interaction forms
A. Immune complex
B. Enzyme complex
C. Hormone complex
D. Lipid complex
Answer: A

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