1. مادة المناعة-النظري\ المرحلة الثالثة
كلية المأمون الجامعة\قسم تقنيات التحليلات المرضية
مادة المناعة-النظري\ المرحلة الثالثة
م.م. رشد اياد عبد الحميد
Lec. 4
Antigen

2. Antigen is a molecule that react with B cell receptor (Antibody) or with T cell receptor (TCR) .
Immunogen is a molecule that induce an immune response; humeral (activation of Bcell to produce Ab) or cellular(activation of T cell by APC).
some antigens bind to Ag-specific receptors but cannot induce an immune response by itself (non-immunogenic) it defined as hapten such as lipid and nucleic acid ; it become immunogenic only when combined with proteins(carrier) or polysaccharide. Hapten: A small organic molecules that are incabable
to induce immune response by themselves ((non-immunogenic) unless complexed with large carrier molecule such as protein and polysaccharide.

3. The antigen may originate from the body (self Ag) or from external environment (non-self Ag).
Vaccines are examples of Ag in an immunogenic form, which are administered to induce the memory function of adaptive immune system toward the Ag.
Allergen is a substance capable of causing allergic reaction after exposure via ingestion, injection or contact with skin.
T- dependent Ag: Ag that require the assistance of T cell to activate B cell to produce specific Ab
T- independent Ag: Ag that stimulate B cell directly without assistance of T cell.

4. Antigens are classified according to their source into:
1.Exogenous antigens ; are antigens that have entered the body from the outside for example, by inhalation, ingestion or injection; taken into antigen presenting cell(APC) by endocytosis or phagocytosis
2. Endogenous antigens ; are generated within host cells as a result of viral or intracellular bacterial infection or produced by cancer cell.

5. Immunogenicity: is the ability of immunogen to activate immune response
There are several properties of immunogen:
Foreignness; the immunogen must be recognized as non- self(foreign) by immune system to induce immune response.
2.Chemical structure; most naturally occurring antigens are proteins and polysaccharide. Proteins(polypeptide) are more effective in stimulating antibody production (proteins is most potent immunogen) while carbohydrates,steroid and lipid are poor immunogens.

6. 3. Molecular size;
• Molecular weight > 100,000 dalton--- immunogenic.
• Molecular weight < 10,000 dalton ---nonimmunogenic.
4. Susceptibility to Ag processing and presentation by APC (displayed with MHC).
The important characteristics of antigen:
Immunogenicity: is the ability of Antigen to activate specific immune response.
2. Antigenicity: is the ability of Antigen to react specifically with Ab and/or T cell-surface receptors.

7. Antigenic determinants (Epitopes): is that portion of antigen that react with the product of specific immune response (antibodies) and T- cell receptors.

8. Epitope structure:
•Size: 4 – 5 amino acids (protein), polysaccharide; •Conformation: Linear or conformational.
If Ag have one epitope is monovalent
If Ag have many epitopes is multivalent

10. Antigen recognized by B- cell are :
a. Proteins, carbohydrates, nucleic acid.
b. Linear and conformational determinants
c. Soluble and membrane
Antigen recognized by T- cell are :
a. peptides
b. Linear determinants
c. on cell membrane within groove of MHC molecule

11. Classification of antigens
Based on immunogenicity
Based on chemical nature
According to whether the help of T-cells produce Ab
Complete Ag which have both immunogenicity and antigenicity
Proteins:the majority of immunogens are proteins (pure proteins or glycoproteins or lipoproteins. Proteins are very good immunogenes.
TD-Ag (thymus dependent Ag)is complex in structure such as proteins, glycoprotein,lipoprotein; it is induce memory response
Incomplete Ag have only antigenicity such as haptens
Polysaccharide and lipopolysaccharide are good immunogenes.
TI-Ag (thymus independent Ag)is simple in structure such as bacterial lipopolysaccharide;and it is not induce memory response
Nucleic acid are poorly immunogenic. However,they become immunogenic when single stranded or when complexed with proteins.
Lipids are non immunogenic , although they may be haptens.