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UNIT 4: ANTIGENS

 ANTIGENS- substances recognized as FOREIGN by the body and have the capability to
react with a complementary antibody or cell receptor but may not evoke an immune
response.
 HOMOLOGOUS ANTIGEN- an antigen that induces an antibody and reacts
specifically with it.
 HETEROLOGOUS ANTIGEN- an antigen that reacts with an antibody it did not
induce (cross-reaction)

 PARTS OF AN ANTIGEN
1. CARRIER PORTION- responsible for the molecular weight
2. EPITOPE/DETERMINANT- determines the specificity of antigen

 IMMUNOGENS- macromolecules, capable of triggering an adaptive immune

response by inducing the formation of antibodies or sensitized T cell in an
immunocompetent host.
-Antigens which are able to elicit ANTIBODY-MEDIATED IMMUNE RESPONSE in a
responsive host.

“IMMUNOGENS ARE ALSO ANTIGENS, BUT NOT ALL ANTIGENS ARE IMMUNOGENS.”

 TWO PROPERTIES OF ANTIGENS

1. IMMUNOGENECITY- inherent ability of a substance to induce specific
response

 FACTORS AFFECTING IMMUNOGENECITY

FOREIGNNESS- must be recognized by the body as “non-self”
 Type of Antigen as to foreignness:
I. AUTOLOGOUS- found on the same individual
II. SYNGENEIC/ISOLOGOUS- found in identical twins
III. ALLOGENEIC/HOMOLOGOUS- found within the same
species but different individual
IV. XENOGENEIC/HETEROLOGOUS- found between different
species
V. SEQUESTERED
VI. TISSUE SPECIFIC ANTIGENS- antigens that are common and
unique in some organs

MOLECULAR SIZE- an immunogen must have a molecular weight of at

least 10, 000 in order to be recognized by the immune system.

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NOTE: INCREASED MOLECULAR WEIGHT=INCREASED IMMUNOGENECITY

CHEMICAL COMPOSITION and MOLECULAR COMPLEXITY- the more

complicated the structure, the more immunogenic
 PROTEIN: most immunogenic; composed of amino acids
 CARBOHYDRATES (polysaccharides)
 GLYCOPROTEINS
 NUCLEIC ACIDS and LIPIDS: least immunogenic

DEGRADABILITY- the immunogen must be degraded and presented to

the cells of the immune system.

ROUTE OF ENTRY- intravenous and intraperitoneal routes are effective;

the intradermal route offers stronger stimulus than the subcutaneous or
intramuscular route.

DOSE- the smaller the dose, the less likely a response.

SOLUBILITY or RETENTION- less soluble antigens are MORE immunogenic

ACCESIBILITY TO REACTIVE SITES- antigenic part must be accessible

- DNA with hidden reactive sites are weak antigens

2. ANTIGENECITY- the ability to react specifically with the antibody or cell that
caused it to be produced.

 RELATIONSHIP OF ANTIGENS TO THE HOST

 AUTOANTIGENS- are antigens that belong to the host.
- do not evoke an immune response under normal circumstances

 ALLOANTIGENS- are from other members of the host’s species, capable of

eliciting an immune response.
-They are important to consider in tissue transplantation and in blood
transfusions.

 HETEROANTIGENS- are from other species, such as other animals, plants, or

microorganisms.

IMMUNOLOGY AND SEROLOGY KRYSTLE VEGA D. MABATAN, RMT

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 HETEROPHILE ANTIGENS- are heteroantigens that exist in unrelated plants or

animals but are either identical or closely related in structure so that antibody
to one will cross-react with antigen of the other.

 HAPTEN- Incomplete antigen; NOT IMMUNOGENIC by itself but when coupled with
a carrier protein, can elicit an immune response.

 ADJUVANT- is a substance administered with an immunogen that increases the

immune response.
- It acts by producing a local inflammatory response that attracts a large number
of immune system cells to the injection site.
-They are thought to enhance the immune response by:
1. Prolonging the existence of immunogen in the area
2. Increasing the effective size of the immunogen
3. Increasing the number of macrophages involved in antigen processing

 EXAMPLES OF ADJUVANTS
COMPLETE FREUND’S ADJUVANT- consists of mineral oil, emulsifier, and
killed mycobacteria (0.5 mg/mL).
- stimulates t cells

LIPOPOLYSACCHARIDES(cell wall of bacteria)- stimulates b cells

ALUM ADJUVANT- Stimulates phagocytes

SQUALINE- adjuvant for HIV vaccine from shark liver oil

IMMUNOLOGY AND SEROLOGY KRYSTLE VEGA D. MABATAN, RMT

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UNIT 5: ANTIBODIES

 ANTIBODIES- are glycoprotein substances developed by PLASMA CELLS in response to the

presence of antigens.
-Found in the serum portion of the blood.
- They play an essential role in antigen recognition and in biological activities related to
the immune response (opsonization and complement activation).

 GENERAL FUNCTIONS:
 Neutralize toxic substances.
 Facilitate phagocytosis and kill microbes
 Combine with antigens on cellular surfaces and thereby cause the destruction
of these cells either extravascularly (outside of the blood vessels within the
mononuclear-phagocyte system) or intravascularly (within the blood vessels
through the action of complement).

 TYPES OF ANTIBODIES
 HETEROANTIBODIES (XENOANTIBODIES)- produced in response to antigens from
another species.
 ALLOANTIBODIES- produced in response to antigens from individuals of the same
species.
 AUTOANTIBODIES- produced in response to body’s own antigens.

 TETRAPEPTIDE STRUCTURE OF IMMUNOGLOBULINS

 The basic structure of immunoglobulins was elucidated in the 1950s and
1960s by the efforts of two men: GERALD EDELMAN and RODNEY PORTER.

 SVEDBERG UNIT indicates sedimentation rate in analytical centrifuge.

 BASIC STRUCTURE: 2 HEAVY CHAINS and 2 LIGHT CHAINS held together by

DISULFIDE BONDS.

LIGHT CHAINS
-Two isotypes:

1. KAPPA
2. LAMBDA
-Antibodies have ONLY ONE type of light chain (never both in
one antibody)
-Kappa:Lambda ratio in normal serum= 2:1

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HEAVY CHAINS
-Five isotypes:
1. Gamma: IgG
2. Alpha: IgA
3. Delta: IgD
4. Mu: IgM
5. Epsilon: IgE
-Antibody classes are defined by their unique heavy chain.

 DOMAINS:
VARIABLE REGIONS- found in the amino-terminal end of each chain.
- Constitute the idiotype of the molecule
- Essential for the formation of antigen binding site
- it is responsible for specificity

CONSTANT REGIONS- found in the carboxy-terminal end

- responsible for various biologic functions. (example: complement
activation and binding to cell surface receptors, placental transfer)
- divided into CH1, CH2, and CH3.
 CH2- binds with complement specifically C1q and initiates
complement pathway
 CH3responsible for cytotropic reactions involving
macrophages and monocytes, mast cells, cytotoxic cells and
B cells.

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 ANTIBODY VARIATIONS
ISOTYPE- same heavy chain for each class, the H chain that is unique to each Ig
class
ALLOTYPE- genetic variations in constant regions
IDIOTYPE- variations in variable regions that give the individual Ig molecules specify

FAB, Fc and HINGE REGION

-A typical monomeric immunoglobulin molecule consists of three
globular regions (two Fab regions and an Fc portion) linked by a
flexible hinge region.
 Fab- Fragment Antigen Binding
 Fc- Fragment crystallization
 HINGE REGION- The segment of H chain located between the
CH1 and CH2 regions
-It has a high content of proline and hydrophobic residues; the
high proline content allows for flexibility.
-only GAMMA, DELTA and ALPHA CHAINS have a hinge region

Fab fragment Fc fragment

-Have antigen-binding -no antigen-binding ability
capacity -spontaneously crystallized at
- each Fab consists of one L 4◦C
chain and one-half of the -represent the carboxy-
one H chain, held together terminal halves of two H chains
by disulfide bonding -important in opsonization and
complement fixation

J (joining) CHAIN- found in IgM (pentamer) and IgA (dimer)

-glycoprotein with several cysteine residues
-serve as linkage points for disulfide bonds between two adjacent
monomers.

ENZYME DIGESTION and FRAGMENTATION OF ANTIBODIES

Scientist Enzyme Result

1. EDELMAN Ab=2 HC+ 2 LC
2. PORTER Papain Ab= 2 Fab + 1Fc
3. NISONOFF pepsin Ab= F(ab)2+ Fc’
4. CRAIG Ab= Constant region +
Variable region

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 FIVE CLASSES OF IMMUNOGLOBULINS

I. IgG- monomer

-predominant immunoglobulin in the adult

-produced in secondary (anamnestic) antibody response
- Best for PRECIPITATION and OPSONIZATION

FUNCTIONS:
• For “anamnestic response” or secondary response
• Only immunoglobulin that can cross the placenta of the mother
and give immunity to the baby or fetus
• Activates the classical pathway of the complement system
• Major opsonizing antibody

SUBTYPES OF IgG:
a. IgG1 (γ1)- dominant IgG subtype in serum and is capable of class
switching (from IgM in the 1st response)
b. IgG2 (γ2)- only IgG that CANNOT cross the placenta
c. IgG3 (γ3)- most effective binder of complement (Ig3,Ig1,Ig2,Ig4)
d. IgG4 (γ4)- DOES NOT activate the complement

II. IgM- pentamer

-known as the “Macroglobulin”

-Most primitive
- It is the 1st immunoglobulin to appear in phylogeny and the last to
appear in senescence
-first to appear after first antigenic stimulus

FUNCTIONS:
• Produced during primary immune response
• It is the most efficient immunoglobulin at activating complement.
• Predominant antibody produced by the fetus; present in colostrum
and mother milk to protect newborn.
• Best for AGGLUTINATION

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III. IgA- monomer in SERUM(IgA1); dimer in BODY SECRETIONS (IgA2)

-predominant immunoglobulin in secretions such as tears, saliva, sweat,
breast milk, respiratory tract, genital and intestinal secretions.

FUNCTIONS:
 Fixes complement through the alternative pathway
 Serves as an opsonin for phagocytosis through a specific FC
receptor on macrophages
 Induces eosinophil degranulation through a specific receptor and
this is responsible for parasitic response of the body.

IV. IgD- monomer

-heat and acid labile
-unknown function
-second antibody to appear on B cell surface
-Primarily a cell membrane surface component of B cells
-Short half-life
-postulated to be with an anti-idiotypic antibody (Ab to Ab) and may be
involved in a feedback mechanism to switch off B cells.

FUNCTIONS:
 Function in immunoregulation
 Major membrane immunoglobulin on the surface of B-lymphocytes
especially in newborns
 Participate in activation of B-cells when there is pathogens

V. IgE- monomer
-was originally called “REAGIN”
-heat-labile

FUNCTIONS:
 elevated during parasitic infections and Type I allergic reactions.
 binds to crystallizable fragment (Fc) receptors on mast cells and
basophils

ANTIBODY SYNTHESIS OCCURS IN THREE PHASES:

1. ANTIGEN ELIMINATION
-this phase is accomplished by phagocytosis.
-most injected antigen is removed within minutes, but complete removal
may take months or years.

2. THE PRIMARY RESPONSE

-occurs in four phases after exposure to antigen

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I. LAG PHASE- no antibody is detectable

II. LOG PHASE- antibody titer increases logarithmically
III. PLATEAU PHASE- antibody titer stabilizes
IV. DECLINE PHASE- antibody is catabolized
-IgM is the first immunoglobulin to appear.
 Although a small amount of IgG is made later, the majority of
immunoglobulin produced during a primary response is IgM.

3. THE SECONDARY RESPONSE

-Elicited by a second or any subsequent exposure to the same antigen
-exhibits the same four phases as the primary response
-there is a rapid antibody response
-IgG is the predominant immunoglobulin.
-Circulating antibody titer is much higher and lasts longer than in primary
response.

NOTE: Please review the Steps of the Monoclonal Antibody Test

IMMUNOLOGY AND SEROLOGY KRYSTLE VEGA D. MABATAN, RMT