2. General Characteristics of
Antigens
Antigens are substances that can induce an immune response in the body.
These can be proteins, polysaccharides, lipids, or nucleic acids. They are
typically found on the surface of pathogens such as bacteria, viruses, or
parasites, but can also be produced by the body's own cells. The immune
system distinguishes between self and non-self antigens, initiating an
immune response against non-self antigens. Antigens are critical for the
body's defense against infections and play a vital role in vaccine
development.
3. Foreignness: Antigens are typically substances that are perceived as foreign to the host organism.
This can include molecules from pathogens such as bacteria, viruses, fungi, and parasites.
Complexity: Antigens are often complex molecules with a three-dimensional structure. They can be
proteins, glycoproteins, polysaccharides, lipids, or nucleic acids. The complexity of antigens allows
the immune system to recognize a wide range of foreign substances.
Specificity: The immune system recognizes antigens based on their specific molecular structures.
Each antigen has distinctive features that can be recognized by immune cells, particularly by B cells
and T cells.
Immunogenicity: Antigens have the ability to induce an immune response. The degree of
immunogenicity varies among different antigens. Some antigens are highly immunogenic and can
elicit a strong immune response, while others may be weakly immunogenic.
4. Antigenic Determinants (Epitopes): Antigens usually have specific regions called antigenic
determinants or epitopes. These are the parts of the antigen that are directly recognized by
antibodies or T cell receptors. An antigen may have multiple epitopes.
Molecular Size: Antigens can vary in size from small molecules to large macromolecules. Larger
antigens, such as proteins, are often more immunogenic because they provide a greater number of
epitopes for recognition by the immune system.
Degradability: Antigens need to be degraded into smaller fragments (peptides) for presentation to
immune cells. This process is essential for the activation of T cells, as they recognize antigens
presented as peptides on the surface of antigen-presenting cells.
Ability to Elicit Memory: Antigens have the capacity to induce immunological memory. Once the
immune system has been exposed to a specific antigen, it can mount a faster and more robust
response upon subsequent encounters, providing protection against future infections
5. Types of Antigens
Exogenous Antigens
These are antigens that enter
the body from the outside, such
as through inhalation,
ingestion, or injection.
Endogenous Antigens
Produced inside the body,
often as a result of cell stress,
viral infection, or cancer.
Autoantigens
These antigens are derived
from normal cellular
constituents, recognized as
"self" by the immune system
under normal conditions.
6. Self-Antigens
1 Immune Tolerance
The body's ability to
recognize self-antigens
as "friend" and not initiate
an immune response
against them.
2 Role in Autoimmune
Diseases
Abnormal immune
response against self-
antigens, leading to
autoimmune conditions
such as rheumatoid
arthritis and lupus.
3 Regulation by T Cells
T cells play a vital role in
recognizing and
regulating the immune
response to self-antigens.
7. Foreign Antigens
1 Pathogen Recognition
Immune system's ability to identify and
mount a response against foreign
antigens introduced by pathogens.
2 Vaccine Development
Utilization of foreign antigens in
vaccines to train the immune system
for future encounters with the
pathogen.
3 Allergen Responsiveness
Some foreign antigens can trigger allergic responses in susceptible individuals.
8. Antigenic Determinants
Epitopes
Specific portions of antigens
that bind to antigen
receptors on immune cells,
triggering an immune
response.
Diversity
Antigens can have multiple
antigenic determinants,
allowing the immune system
to recognize a wide range of
pathogens.
Importance in Vaccines
Understanding and targeting
antigenic determinants is
crucial in the development of
effective vaccines.
9. Immunogenicity of Antigens
Factors Affecting Immunogenicity
Antigen size, complexity, and route of administration influence the strength of the
immune response.
Adjuvants
Substances added to vaccines to enhance their immunogenicity and improve the
body's response to antigens.
Variability
Different antigens elicit varying degrees of immune response due to their unique
immunogenic properties.
10. Antigen Processing and Presentation
Antigen Processing The breakdown of antigens into smaller
peptides, often performed by specialized
immune cells.
Antigen Presentation Display of antigen-derived peptides on the
surface of cells for recognition by T cells.
11. Major Histocompatibility Complex (MHC)
1
MHC Class I
Found on all nucleated cells. Presents
antigens from inside the cell to cytotoxic
T cells. 2 MHC Class II
Expressed on antigen-presenting cells.
Displays antigens from outside the cell to
helper T cells.
13. Clinical Significance of Antigens
1 Disease Diagnosis
Antigens are used in diagnostic tests
to detect the presence of specific
pathogens or abnormal cells.
2 Transplantation
Matching antigens between donor and
recipient is crucial to minimize
rejection in organ transplantation.
3 Immunotherapy
Targeting antigens to stimulate or suppress specific immune responses in the treatment
of diseases.
