2. Learning Outcomes
• Explain antibody function, and describe the
structural and chemical characteristics of
antibodies.
• Name one function for each of the five classes of
antibodies
• Define antigen, epitope, and hapten.
3. Antigen and its characteristics
Antigen: are substances that induce production of
antibodies (and often known as immunogens).
Common types of antigens:
1. microbial antigen
a.Bacterial antigens: can have multiple antigenic
sites such as e.g. cell wall (O), flagellar (H),
capsular and fimbrial antigens. Also
extracellular antigens (e.g. exotoxins) and
intracellular antigens (e.g. nucleoproteins).
b.Other microbial antigens: such as viral,
parasitic and fungal antigens.
4. • Each Y-shaped antibody molecule has two binding
sites that attach to a specific regions on antigens
called epitopes (antigenic determinants) of cell wall
an antigen.
5. Most antigens are either :
- proteins or large polysaccharides.
- Lipids and nucleic acids are usually antigenic only
when combined with proteins and polysaccharides
• Note:-
• Pathogenic bacteria possess a number of
recognizable antigens called pathogen-associated
molecular patterns (PAMPs). PAMPs serve as
warning flags of an invading organism that the host
can recognize by means of receptors. The best-
known of these receptors is the extended family of
Toll-like receptors (TLRs).
6. 2.Tissue antigens: (e.g. blood group antigens).
3.Drugs: some drugs act as antigens while others act
as haptens.
4.Food antigens: such as proteins of milk, egg and
other foods.
7. Cross-reactive antigens : are those antigens which
share one or more identical or similar epitopes,
therefore an antibody formed to any of them could
react with the other. Examples are:
• Human heart valve tissue and certain antigen
found in Streptococcus pyogenes.
8. Factors of affecting
immunogenicity of antigen
Factors related to antigen
1. Foreignness
Foreignness means substances that never contact with
lymphocytes in embryo period.
The immune system somehow discriminates (recognize)
between "self" and "non-self" so that only molecules
that are foreign to the circulation of an animal are
immunogenic. The more foreignness the more
powerful the antigen is.
9. 2) Molecular size:
small molecules such as monosaccharides and
amino acids are not immunogenic, while
proteins and polysaccharides are immunogenic.
As a general, substances with molecular weight
over 100,000 are potent immunogenic and
those around 10,000 daltons are weak
antigenic.
(3) Complexity and chemical structure
The more complexity of the molecule, the more
the antigenicity, (proteins more antigenic than
carbohydrates).
Protein> Polysaccharides, nucleic acids, lipids.
Polymer > Monomer.
Cycle molecule >linear molecule.
10. 4) Specificity:
• Although strong antigens are large molecules, only
restricted portions of them are involved in the actual
binding with the antibody combining sites.
•Such areas determine the specificity of the antigen
and are designated as antigenic determinants
(epitopes).
(5)Route of administration:
Oral route of administration of antigen is less than
subcutaneous, intravenous or intramuscular
concerning the stimulation of immune response
11. • Auto-antigen: it is self-antigen (normal constituent
of the body tissue).
• Iso-antigen: it is an antigen in an individual of one
species which is capable of eliciting an immune
response in genetically different individual of the
same species e.g. RBCs (blood grouping) and tissue
antigens.
According to recognition by Immune system, antigen
classified into:
• T-cell dependent Ag: - Requires T cell help for B cell
activation; e.g. proteins (microbial proteins & non-
self or altered-self proteins ).
• T-cell independent Ag (TI): - activate B cells without
help from T cell ; e.g. polysaccharides, LPS.
12. Activation of B cells to produce antibodies.
• the B cell is producing antibodies against a T-
dependent antigen.
This process is called clonal expansion (proliferation)
13. • T-independent antigens. T-independent
antigens have repeating units (epitopes) that can
cross-link several antigen receptors on the same B
cell. These antigens stimulate the B cell to make
antibodies without the aid of T helper cells. The
polysaccharides of bacterial capsules are examples
of this type of antigen
14. Depending on the nature of immune
responses, antigens are divided into 3
fuctional types:
a. Immunogens
- Large Ag with epitopes capable of binding
with immune receptor & inducing immune
response. (Notice that not all antigens are
immunogens.
b. Tolerogens
Self Ag (MHC) normally not stimulate immune
system
15. c. Haptens
• A small Ag have a molecular mass less than 10,000
da with epitopes capable of binding with immune
receptor & without inducing immune response .
• To induce immune response to the hapten, it
require to be attached to a large carrier molecule
(usually serum protein). Penicillin is a good
example
of a hapten
