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1. IMMUNITY
Immunity is the resistance exhibited by the
host towards the injury caused by micro-
organisms and their products.

2. IMMUNITY
ACTIVE PASSIVE
INNATE/NATIVE ACQUIRED
NATURAL ARTIFICIAL ARTIFICIAL
NATURAL

3. INNATE IMMUNITY
Definition-Innate or native immunity is the
resistance to infections that an individual
possesses by virtue of his /her genetic and
constitutional makeup.

4. INNATE IMMUNITY- 3 TYPES
• Species immunity-total or relative
refractoriness to a pathogen shown by all
members of a species.
• Racial immunity-different races may show
difference in susceptibility to infection.
• Individual immunity-different individuals in a
race show difference in immunity.

5. FACTORS AFFECTING INNATE
IMMUNITY
• Age-very young and very old are more
susceptible to infections.
• Hormones-endocrine disorders enhances
susceptibility to infection
• Nutrition- Immunity is reduced in
malnourished conditions.

6. COMPONENTS OF INNATE IMMUNITY
1) Epithelial surfaces
Epithelial cells lines all the sites of entry of microbes
and act as a mechanical barrier.
• Skin possess bactericidal activity due to high
concentration of salt (high Ph) in the sweat.
• Sebaceous glands secrete sebum containing long chain
fatty acids.
• Lysozyme an enzyme present in the skin dissolves
bacterial cell wall.
• Epithelial cells produce defensins which kill microbes.

7. CONJUNCTIVA
• Lachrymal secretion has the flushing action
• Tears contain the antibacterial substance
called lyzosyme

8. MUCOSAL SURFACES
• Respiratory tract-
• Architecture of nose prevents the entry of
microorganisms.
• Those pass beyond are held by mucociliary
apparatus, swept back to the pharynx,
swallowed or coughed out.
• Cough reflex is an important defence
mechanism of the respiratory tract.
• Particles that reach the pulmonary alveoli are
ingested by the phagocytic cells.

9. GASTRO-INTESTINAL TRACT
• Enzymes present in saliva kills bacteria.
• High acidity of stomach destroys bacteria.
• Small intestine secretes proteolytic enzymes.

10. Urinary tract-flushing action of urine

11. MICROBIAL ANTAGONISM
The skin and mucosal surfaces have resident
bacterial flora which prevents colonization by
pathogens.
Lactobacillus (normal flora) in the vagina
maintains the acidic pH which prevents the
entry of microbes.

12. • Fever
• Inflammation
• Phagocytic cells- Macrophages and microphages
• Natural killer cells-a class of lymphocytes which
destroys tumor cells and viral infected cells.
• Complement system-enhances phagocytosis ,
promotes inflammation.

13. ACQUIRED IMMUNITY
Definition-the resistance that an individual
acquires during life is known as acquired
immunity.
2 types
1.ACTIVE IMMUNITY
2.PASSIVE IMMUNITY

14. ACTIVE IMMUNITY is developed by an individual
as the result of stimulation by an antigen. This
response is also known as adaptive immunity.
PASSIVE IMMUNITY is the resistance that is
transmitted passively to a recipient in a
readymade form is known as passive immunity.
Here recipient’s immune system plays no role.

15. ACTIVE IMMUNITY
• Involves an active response by the host and
results in the production of various substances
and immune cells.
• Occurs only after a lag period which is required
for the immune system to react.
• It is long lasting. Secondary response is stronger
and quick.
• It is associated with immunological memory.
• It is more effective and gives better protection.

16. Active immunity is classified into;
Natural active immunity-results from a clinical
or an inapparent infection by a microbe. For
many viral diseases immunity is lifelong than
bacterial diseases.
Artificial active immunity-this is the resistance
induced by vaccines.

17. PASSIVE IMMUNITY
• There is no antigenic stimulus, preformed
antibodies are administrated.
• There is immediate protection, no lag period
• There is no secondary response.
• Not associated with immunological memory.
• The response is less effective than active
immunity.

18. Passive immunity is classified into 2 ;
Natural passive immunity-this is the resistance
passively transferred from the mother to baby.
In human infants, maternal antibodies are
transmitted predominantly through placenta.
The human colostrum , which is also rich in IgA
antibodies , gives protection to the neonate.
Artificial passive immunity-this is the
resistance passively transferred to a recipient
by the administration of antibodies.

19. HERD IMMUNITY
Herd immunity refers to the overall level of
immunity in a community and is relevant in the
control of epidemic diseases
 When a large proportion of individuals in a
community are immune to a pathogen, the herd
immunity to the pathogen is satisfactory.
 When herd immunity is low, epidemics are likely
to occur when a suitable pathogen is introduced,
due to the presence of large number of
susceptible individuals in the community.

20. • Eradication of communicable diseases
depends upon the development of high level
of herd immunity rather than high level of
immunity in individuals.
• Herd immunity can be increased by
vaccination of large numbers of people in the
community.

21. ANTIGENS AND ANTIBODIES

22. ANTIGEN
An antigen can be defined as any substance
which when introduced parenterally,into the
body ,stimulates the production of an
antibody with which it reacts specifically and
in an observable manner.

23. CHARACTERISTICS OF AN ANTIGEN
• Induction of an immune response-
immunogenicity
• Specific reaction with antibodies or sensitised
cells-immunological reactivity.

24. PROPERTIES OF ANTIGENS
• Chemical nature-proteins are more antigenic
• Size-large molecular size –more antigenic
• Foreignness.
•

25. ANTIBODIES/IMMUNOGLOBULINS
• Antibodies or immunoglobulins (Ig) are
glycoproteins that is made in response to an
antigen, and can recognize and bind to the
antigen.

26. STRUCTURE OF AN
IMMUNOGLOBULIN

27. • 2 light chains (L chains)
• 2 heavy chains (H chains)
• H chain is attached to L chains by a disulphide
bond.

28. HEAVY CHAINS
The immunoglobulins are classified on the basis
of H chains.
There are 5 classes of H chains and therefore 5
classes of immunoglobulins.
IgG, IgA,IgM,IgD,IgE

29. LIGHT CHAINS
• similar in all classes of immunoglobulins
• Occur in 2 varieties-
KAPPA(Ƙ)
LAMBDA(ƛ)

30. SPECIFICITY
Each individual antibody will only binds to one
antigenic determinant.

31. ANTIBODIES
Human sera contains;
IgG
IgA
IgM
IgD
IgE

32. IgG
• major immunoglobulin in human serum, accounting for
about 80 percent of the total immunoglobulin pool.
• It has a sedimentation coefficient of 7S and a molecular
weight of 150,000.
The normal serum concentration of IgG is about 8 to 16
mg per ml.
• It has a half-life of 23 days
• IgG is the predominant immunoglobulin in blood, lymph,
peritoneal fluid, and cerebrospinal fluid, and it is
distributed nearly equally between extra-and intravascular
spaces.

33. FUNCTIONS OF IgG
 general purpose antibody, protective against those
infectious agents which are active in the blood and tissues.
 Transfer from mother to foetus: IgG is the only class of Igs
that can cross the placenta and is responsible for the
protection of the infant.
 Opsonization: IgG binds to microorganisms and enhances
their phagocytosis.
 Immunological reactions: IgG participates in most
immunological reactions such as complement fixation,
precipitation and neutralization of toxins and viruses.
 Immobilize bacteria: IgG can also immobilize bacteria by
binding to their cilia or flagella.

34. IgA
• It is the second most abundant class, constituting
about 10 to 13 percent of serum
immunoglobulins.
• The normal serum level is 0.6 to 4.2 mg per ml.
• It has a half-life of 6-8 days.
• IgA is the primary immunoglobulin found in
external secretions, such as mucus, tears, saliva,
gastric fluid, colostrum and sweat.

35. IgA occurs in two forms.
• Serum IgA -Serum IgA is monomeric 7S
molecule (MW about 160,000).
• secretory IgA (SIgA)-IgA found on mucosal
surfaces and in secretions is a dimer formed
by two monomer units joined together at
their carboxy terminals by a glycopeptide
termed the J chain .

37. FUNCTIONS OF IgA
 Local immunity: Secretory IgA (SIgA) is
selectively concentrated in secretions and on
mucus surfaces forming an ‘antibody paste’ and
is believed to play an important role in local
immunity against respiratory and intestinal
pathogens.
Prevention of organisms entry into body tissues
Promotes phagocytosis
Promotes intracellular killing of micro organisms.
Agglutination: It can cause agglutination, and can
also prevent viruses from entering cells.

38. (IgM)
Constitutes 5-8% of serum immunoglobulins
It is a heavy molecule (19S; MW 900,000 )
hence called ‘millionaire molecule’).
The normal serum level of IgM is 1.2 mg/ml.
It has a half-life of about 5 days.
IgM is the first immunoglobulin to appear
after exposure to an antigen.

39. IgM
In the circulation, IgM exists as a pentamer of
five four-chain units. The five identical IgM
monomers are connected to each other by a
polypeptide joining J chain.
• 10 antigen-binding sites.
• IgM (80 percent) is intravascular in
distribution

41. relatively short-lived hence their
demonstration in the serum indicates recent
infection.
FUNCTIONS OF IgM
Being largely confined to the intravascular
spaces,IgM is believed to be responsible for
protection against blood invasion by
microorganisms.
Monomeric IgM is present on the surface of
mature, naive B cells for antigen recognition.

42. IgD
Its molecular weight is 180,000 daltons.
IgD is an immunoglobulin found in trace
amounts in the blood serum (0.03 mg/ml).
Half-life is about 3 days.
IgD antibodies are abundant in combination
with IgM on the surface of B cells and bind
antigens, thus signalling the B cell to start
antibody production.

43. IgE
o It resembles IgG structurally and also known
as reagin antibody.
o IgE is an 8S molecules (MW 19,000) and half-
life of two days.
o It is present in extremely low amounts in
serum.
o IgE does not activate complement nor
agglutinate antigens.

44. IgE-ALLERGIC REACTIONS
IgE molecules bind tightly to receptors on mast
cells and basophils, specialized cells that
participate in allergic reactions. IgE may be
elevated in allergic (atopic) individuals, and is
responsible for many of the symptoms of
allergies, bronchial asthma and even systemic
anaphylaxis.

45. ANTIGEN- ANTIBODY REACTIONS

46. Antigen-antibody reactions in vitro are known as
serological reactions.

47. A. Precipitation reactions
B. Agglutination reactions
C. Complement fixation test (CFT)
D. Neutralization tests
E. Opsonization
F. Immunofluorescence
G. Radioimmunoassay (RIA)
H. Enzyme-linked immunosorbent assay (ELISA)
I. Immunoelectroblot techniques
J. Immunochromatographic tests
TYPES OF ANTIGEN AND ANTIBODY REACTIONS

48. PRECIPITATION REACTIONS
• When a soluble antigen combines with its
specific antibody at a suitable temperature
and pH, the antigen antibody complex will
form as an insoluble precipitate in a reaction is
known as a precipitation.
• When instead of settling, the precipitate
remains suspended as floccules, the reaction
is known as flocculation.

49. 1) If there is more antibody than antigen, only a
week precipitation will occur-prozone(zone of
antibody excess)
2) If there is more antigen than antibody, only a
week precipitation will occur-postzone(zone of
antigen excess)
3) If the amount of antigen and antibody are
similar, largest reaction occurs-zone of
equivalence
ZONE PHENOMENON

50. Type of precipitation reactions
1) RING TEST
Simplest type, antigen solution is layered over a
column of antiserum in a narrow tube.
A precipitate is formed at the junction of two
liquids.
Application-grouping of streptococci

52. 2) SLIDE TEST
A drop of antigen and antiserum are placed in a
slide and mixed by shaking, floccules appear.
Application- VDRL TEST FOR SYPHILIS

53. 3) TUBE TEST
Antigen solution is mixed with antibody
solution in a tube; floccules appear than
remain suspended in solution.

54. IMMUNODIFFUSION
• These are precipitation tests in gel .
Immunodiffusion is performed in soft agar (1%
agar) or (agarose gel)
Immunodiffusion is classified into 2
1)Single diffusion-either antigen or antibody is
moving.
2)Double diffusion-both antigen and antibody
are moving.

55. 1. SINGLE DIFFUSION IN ONE DIMENSION
Antibody is incorporated in gel in a test tube
and the antigen solution is layered over it.
The antigen diffuses downward through the
agar gel forming a line of precipitation, where
it meets the antibody.

57. 2. DOUBLE DIFFUSION IN ONE DIMENSION-
The antibody is incorporated in the gel, above
which is placed a column of plain agar. The
antigen is layered on top of this. Antigen and
antibody moves towards each other through
the plain agar between them and form a band
of precipitate where they meet at the zone of
equivalence.

59. 3. SINGLE DIFFUSION IN TWO DIMENSIONS
Also called radial immunodiffusion
The antiserum is incorporated in agar gel poured
on a flat surface. The antigen is added to the
wells cut into the surface of the gel. The
antigen diffuses from the well and forms rings
of precipitation around the well.

62. • 4. DOUBLE DIFFUSION IN TWO DIMENSIONS
(Ouchterlony procedure)
Most widely used method
Agar is poured on a slide and wells are cut. The
antiserum is placed in the central well and
different antigens are placed in the surrounding
wells. Antigen and antibody diffuses. A line of
precipitate forms between the antigen and the
antibody when the two react.

63. • If the two adjacent antigens are identical, the
lines of precipitate formed by them will fuse.
• If they are unrelated, the lines will cross each
other.

65. IMMUNOELECTROPHORESIS
Immunoeletrophoresis combines electrophoresis
and immunodiffusion. this is done on a glass slide
layered with semisolid agar.a well is cut and antigen
is filled.
The first step is electrophoresis of antigen for about
an hour.
Rectangular trough is cut in the agar parallel to the
direction of migration of antigen and filled with
antibody.
Diffusion is allowed to proceed for 18-24 hours.

66. • Precipitation lines develop with each
separated component of the antigen.by this
technique, a number of antigens can be
identified in human serum.
• It is particularly useful for detection of normal
and abnormal serum proteins like myeloma
proteins.

69. ELECTROIMMUNODIFFUSION
Immunodiffusion can be speeded up if antigen and antibody are driven by electricity.it
is combination of eletrophoresis and diffusion.
They of two types
COUNTER IMMUNOELECTROPHORESIS (CIE)
ROCKET ELECTROPHORESIS
Counter immunophoresis is one dimensional double
eletroimmunodiffusion.
This test is based on movement of antigen and antibody in opposite
direction.
This is performed on a glass slide layered with agarose and wells are cut
on the surface.
One well is filled with antigen and other with antibody . electric current is
passed.
It is clinically applied for detecting hepatitis B antigens and antibodies.
Antigens of Cryptococcus in CSF and in number of other diseases.

70. COUNTER
IMMUNOELECTROPHORESIS (CIE)

71. ROCKET ELECTROPHORESIS

72. • It is one dimensional single eletroimmunodiffusion.
• This technique is mainly applied for quantitation of
antigens . the antibody to antigen to be quantitated is
mixed in agarose and gelled on the glass slide.
• The wells are punched in the set gel
• and filled with increasing concentrations of the antigen.
• It is the electrophoresed.
• Precipitation is formed in the shape of cone like structures.
• The length of these rocket like structures corresponds to
the concentration of the antigen.

75. AGGLUTINATION REACTIONS

76. Agglutination
• It is an antigen- antibody reaction,in which a
insoluble antigen combines with its antibody
in the presence of eletrolytes at an optimal
temperature and pH ,resulting in visible
clumbing of particles.
• Agglutination is more sensitive than
precipitation for detection of antbodies.

77. Types of agglutination reactions
• 1. Slide Agglutination
• A drop of antiserum is added to a smooth
uniform suspension of particulate antigen.
• Result- AGGLUTINATION-clumping together of
the particles and clearing of the fluid.
• Uses
• It is routine procedure to identify the bacterial
stains isolated from clinical specimens.
.it is also used for blood grouping and cross
matching.

80. 2. Tube Agglutination
• Presence of clearing and clumping in tubes
indicates agglutination.
• Serum is diluted serially by doubling dilusion
in test tubes.
• An equal volume of a particulate antigen is
added to all tubes.
• The highest dilution of serum at which
agglutination occurs is antibody titre.

81. • Uses of tube agglutination test
• Serological diagnosis of
• Enteric fever (widal test)
• Typhus fever(weil-Felix reaction.)
• Infectious mononucleosis(Paul-Bunnel test)
• Brucellosis.
• For diagnosis of primary atypical
pneumonia.(streptococcus MG agglutination test)

83. 4. Passive (Indirect) Agglutination Test
A precipitation reaction can be converted into
agglutination reaction by coating soluble
antigen on to the surface of carrier particles.
Such test is more convenient and more
sensitive for detection of antibodies. Such
tests are known as passive agglutination tests.

84. 3. Antiglobulin (Coombs’) test

86. 3.The antiglobulin (coombs) test
• This test was devised originally by coombs ,
Mourant and Race for the detection of
incomplete anti-Rh antibodies .when sera
containing incomplete anti-Rh antibodies are
mixed with corresponding Rh-positive
erythrocytes , the incomplete antibody globulin
coats the erythrocytes but no agglutination
occurs . when such coated erythrocytes are
treated with antiglobulin or coombs serum(rabbit
antiserum against human gamma globulin),the
cells are agglutinated .this is the principle of
coombs test.

88. • There are two types
• Direct coombs test
• Indirect coombs test
• The only difference between the two is that the
sensitisation of the erythrocytes with incomplete
antibodies takes place in vivo in direct type whereas it
occurs in vitro in indirect type.
• Uses of coombs test
• A. for detection of anti-Rh antibodies.
• For demonstration of any type of incomplete antibody
eg. brucellosis.

89. 4.Hetrophile agglutination test
• Heterophile antibodies have a property to
react with microorganisms or cells of
unrelated species due to common antigenic
sharing.
• i.Weil-Felix reaction
• Some proteus strains are agglutinated by sera
of patients with rickettsial infections . this is
due to antigenic sharing between these
proteus strains and Rickettsial species.

90. • II.Paul -Bunnel test
• Sheep erythrocytes are agglutinated by sera of
infectious mononucleosis.
• Streptococcus MG agglutination test
• It is positive in primary atypical pneumonia.

91. 5.Passive agglutination test
• A precipitation reaction can be converted into agglutination
test by attaching soluble antigens to the surface of carrier
particles such as latex particles , bentonite and red blood
cells . such tests are called passive agglutination tests. this
conversion is done because agglutination tests are more
sensitive for detection of antibodies.
• Passive agglutination tests are very sensitive.
• When instead of antigen , the antibody is absorbed on the
carrier particles for estimation of antigens ,it is known as
reversed passive agglutination.
• Latex agglutination test
• Haemagglutination test
• Coagglutinatination

92. Complement Fixation Text(CFT)
• 1.Principle
• the antigen-antibody complexes have ability to fix complement .
this reaction has no visible effect.
• To detect the fixation of complement,an indicater system
consisting of sheep erythrocytes coated with amboceptor (rabbit
antibody to sheep erythrocytes) is used.
• Complement can lyse these erythrocytes coated with antibodies.
• If complement is fixed and utilized in the antigen-antibody
reaction,there is no free complement to act on the indicater system
and hence no lysis of erythrocytes,which indicates the positive
complement fixation test.
• Lysis of erthrocytes indicates that complement was not fixed in the
first step and therefore,the serum is negative for antibodies.

93. • 2.PROCEDURE
• The serum (to be tested) should be inactivated by heating at 56 c
for 30 minutes to destroy any complement activity the serum may
have and to remove some non-specific inhibitors of complement.
• The antigen may be used as soluble or particulate.
• Fresh guinea pig serum is used as source of complement.
• Complement activity is heat labile so it is used as fresh.
• Controls should be included in the test.
• Antigen and serum controls are included to ensure that they are not
anti-complementary.
• Complement control is used to ensure that the desired amount has
been added , and cell control to make sure that sensitised
erythrocytes do not undergo lysis in the absence of complement.

94. NEGATIVE CFT

95. POSITIVE CFT

96. Indirect complement fixation test
• Certain avian (eg . duck , parrot)and
mammalian(eg . horse , cat) sera cannot fix
guinea pig complement.
• Indirect complement fixation test is used for
testing such sera.
• Test is done in duplicate and after the first
step , the standard antiserum known to fix
complement is added in one set.

97. NEUTRALIZATION TEST
• Bacterial exotoxins are capable of producing
neutralising antibodies(antitoxins) which play
a role in protection against diseases such as
diphtheria and tetanus.
• The toxicity of bacterial endotoxins is not
neutralized by antisera.
• Virus may also be neutralized by their
antibodies and these are named as virus
neutralization tests.

98. • Toxin-antitoxin neutralization can be
measured in vivo and in vitro.
• In vivo tests
• 1.toxigenicity test
• Toxigenicity test is done for detection of toxin
of C.diphtheriae.
• It is an intradermal test to inject bacterial
toxin in animal previously protected by
antitoxin of C.diphtheriae.

99. • 2.shick test
• This is similar kind of test in humans ,
diphtheria toxin is injected intradermally in
man , there is no reaction at the site of
injection if person is immune to diphtheria ie
antitoxin is present in blood . Injected toxin is
neutralized by circulating antitoxin.

100. In vitro tests
1.Antistreptolysin o (ASO) test.
Serum of patients suffering from streptococcus
pyogenes infection contains antistreptolysin o
(antitoxin) which neutralizes the hemolytic
activity of streptococcal o haemolysin(toxin).
2.Virus neutralization tests
Neutralization of viruses can be demonstrated in
cell cultures , eggs and animals .it is used mostly
in typing viral isolates.

101. • 3.Nagler reaction
• Clostridium welchii toxin (alphatoxin) is
neutralized by antitoxin when the bacteria is
grown in egg yolk medium containing
antitoxin.it is useful for rapid detection of
c.welchii in clinical specimens.

102. OPSONISATION
• opsonisation is the process by which a
particulate antigen becomes more susceptible
to phagocytosis . this occurs by help of
opsonin which combines with an antigen and
facilitates phagocytosis . Opsonin may be an
antibody like substance or other component
present in serum.

103. Immunofluorescence
• Fluorescence is the property of certain dyes
which absorb rays of one particular
wavelength (ultraviolet light)and emit rays
with a different wavelength (visible light).
• Immunofluorescence test is of two types.
• Direct immunoflurescence test.
• Indirect immunoflurescence test.

104. Direct immunoflurescence test.
• Principle
• The specific antibodies tagged with
fluorescent dye (LABELLED antibodies) are
used for the detection of unknown antigen in
a specimen.
• If antigen is present ,it reacts with labelled
antibodies and fluorescence can be observed
under ultraviloet light of fluorescent
microscope.

105. • Uses
• It is commonly employed for detection of
bacteria ,viruses or other antigens in blood , CSF,
urine , faeces , tissues and other specimens.
• It is sensitive method to diagnose rabies by
detection of the rabies virus antigen in brain
smears.
• Disadvantages
• Separate specific flurescent labelled antibody has
to be prepared against each antigen to be tested.

106. Indirect immunofluorescence test
• The indirect method is employed for detection of antibodies in
serum or other body fluids.
• Principle
• A known antigen is fixed on a slide .
• The unknown antibody(serum) is applied to the slide
• If antibody (globulin)is present in the serum,it attaches to known
antigen on the slide.
• For detection of this antigen-antibody reaction ,flurescein-tagged
antibody to human globulin is added.
• In positive test ,flurescence occurs under ultraviloet light ,
• One specific example of detecting antibodies in serum of syphilis
patient is also included.

107. • Advantages
• A single antihuman globulin florescent
conjugate can be employed for detection of
antibody to any antigen.

108. Radioimmunoassay (RIA)
• Berson and Yallow first described the test
radioimmunoassay and since then it has been
utilized for quantitation of hormones , drugs ,
hepatitis B surface antigen,
Ig E and viral antigens .This test can be detect
antigens up to picogram quantities .

109. • RIA is based on competition of fixed amounts of
specific antibody between a known radiolabelled
antigen and unknown unlabelled antigen.
• This competition is determined by the level of the
test antigen present in the reacting system.
• After antigen antibody reaction the antigen is
separated into free and bound fractions and
their radioactivity is measured.
• The concentration of test antigen is calculated
from the ratio of the bound and total antigen
labels , using a reference curve.

110. • The ratio of bound:total labels (B:T
ratio)plotted against the unlabelled antigen
concentrations gives the standard refernce
curve.
• The concentration of antigen in the test
sample is calculated with the help of B: T ratio
of the test by using standard dose response
or refernce curve.

111. Enzyme linked immunosorbent
assay(ELISA)
• ELISA has been applied for the detection of
variety of antibodies and antigens .
• It requires only microlitre quantities of test
reagents.
• The principle of ELISA is almost same as that of
immunoflurescence,the only difference being ,an
enzyme is used instead of fluorescent dye.
• The enzyme act on substrate to produce a colour
in a positive test.
• It is done on a solid phase.

112. • When an enzyme is linked to an antibody and
used to detect and measure other antibodies or
antigens, the assay is called the enzyme-linked
immunosorbent assay (ELISA).
• An enzyme conjugated with antibody reacts with
a colorless substrate to generate a colored
reaction product.
• Enzyme-linked immunosorbent assay is highly
sensitive, highly specific and less expensive
technique used in serology to detect antigens or
antibodies.

113. Solid phase immunoassay are more widely
used. It refers to the binding of either antigen
or antibody to a variety of solid materials,
such as polyvinyl or polycarbonate wells or
membranes of polyacrylamide , paper or
plastic or metal beads or some other solid
matrix.
IMMUNOSORBENT

114. • The test is usually done using microtiter plates
(96-well) suitable for automation.

116. TYPES OF ELISA
DIRECT ELISA

117. INDIRECT ELISA

119. Sandwich ELISA

120. ANTIBODY CAPTURE ELISA