This document summarizes key concepts in immunology. It defines antigens as substances recognized by B cells or T cells, and antibodies as antigen-binding proteins found on B cells and secreted by plasma cells. The immune system protects organisms from pathogens through innate immunity involving anatomical, physiological, phagocytic and inflammatory barriers, as well as adaptive immunity providing antigen specificity, diversity, immunological memory and self/non-self recognition via T and B lymphocytes. The complement system is a major effector of humoral immunity involving serum proteins that opsonize pathogens and induce inflammatory responses.

1. IMMUNOLOGY

2.  Antigen:
 The substances that can be recognised by the
immunoglobulin receptor of B cells,
or by the T-cell receptor, are called antigens.
 Antibodies:
 Antibodies are the antigen binding proteins present on the B-
cell membrane and
secreted by plasma cells

3.  Antigenicity is the ability to combine specifically with the
antibodies and/or cell
surface receptors
 Immunogenicity is the ability to induce a humoral and/or cell-
mediated immune
response
 A substance that induces a specific immune response is usually
called an immunogen All molecules that have the property of immunogenicity also have
the property of
antigenicity, the reverse is not true
 Some small molecules, called haptens, are antigenic but incapable,
by themselves,
of inducing a specific immune response. In other words, they lack
immunogenicity
 Proteins are the most potent immunogens, with polysaccharides
ranking second.
 In contrast, lipids and nucleic acids of an infectious agent generally
do not serve as
,

4.  The immune system is defense system that has evolved to protect
animals from
invading pathogenic microorganisms and cancer
 An immune response can be divided into two related activities—
recognition and
response
 Humoral immunity refers to immunity that can be conferred upon a
nonimmune
individual by administration of serum antibodies from an immune
individual.
 Cell-mediated immunity can be transferred only by administration of T
cells from
an immune individual.
 Immunity—the state of protection from infectious disease
 Innate immunity
 Provides the first line of defense against infection
 Less specific component
 Innate immunity can be seen to comprise four types of defensive
barriers:
anatomic, physiologic, phagocytic, and inflammatory

6.  Adaptive immunity is capable of recognizing and selectively eliminating
specific
foreign microorganisms and molecules. Antigenic specificity
 Diversity
 Immunologic memory
 Self/nonself recognition
 An effective immune response involves two major groups of cells: T
lymphocytes and
antigen-presenting cells
 Lymphocytes are one of many types of white blood cells produced in the
bone
marrow by the process of hematopoiesis
 Lymphocytes leave the bone marrow, circulate in the blood and lymphatic
systems,
and reside in various lymphoid organs
 The two major populations of lymphocytes—B lymphocytes (B cells) and T
lymphocytes (T cells)

7.  T LYMPHOCYTES
 T lymphocytes also arise in the bone marrow
 T cells migrate to the thymus gland to mature
 During its maturation within the T cell comes to express a unique antigen-
binding
molecule, called the T-cell receptor, on its membrane.
There are two well-defined subpopulations of T cells: T helper (TH) and T
cytotoxic
(TC) cells..
T cells displaying CD4 generally function as TH cells, whereas those
displaying CD8
generally function as TC cells
 B LYMPHOCYTES
 B lymphocytes mature within the bone marrow
 Each expresses a unique antigen-binding receptor on its membrane
 This antigen-binding or B-cell receptor is a membrane-bound antibody
molecule
 Naive B cell-Memory B cells and Effector B cells called plasma cells.

10.  The complemet system is the major effector of the humoral branch of the
immune
system
 Complement includes a collection of serum glycoproteins
 The complement system is made up of a large number of distinct plasma
proteins
circulating in the blood and tissue fluids that react with one another to
opsonize
pathogens and induce a series of inflammatory responses that help to fight
infection
Complement System

11. The Complement Components
 Proteins and glycoproteins
 Synthesized mainly by liver hepatocytes, blood monocytes, tissue
macrophages,
and epithelial cells of the gastrointestinal tract
 Constitute 5% (by weight) of the serum globulin fraction
 Most of them circulate in the serum in functionally inactive forms as
proenzymes
 Complement components are designated by numerals (C1–C9), by letter
symbols
(e.g. Factor D, Properidin)

12. C3
C3a
Small Fragment
C3b
Larger Fragment
Factor B Ba + Bb
 Peptide fragments formed by activation of a component are denoted by small
letters
 In most cases, the smaller fragment resulting from cleavage of a component is
designated “a” and the larger fragment designated “b”
(Note that C2 is an exception: C2a is the larger cleavage fragment
 The complement fragments interact with one another to form functional
complexes.
Those complexes that have enzymatic activity are designated by a bar over the
number
or symbol (e.g., C4b2a, C3bBb)
C4b2aC4b C2a

13.  Lysis of cells, bacteria, and viruses
 Opsonization, which promotes phagocytosis of particulate antigens
 Binding to specific complement receptors on cells of the immune system,
triggering
specific cell functions, inflammation, and secretion of immunoregulatory
molecules
 Immune clearance,which removes immune complexes from the circulation
and
deposits them in the spleen and liver
 Basic functions of Complement

14.  Complement Activation
 Classical pathway
 Alternative pathway
 Lectin pathway
 Phases in Complement cascade
 Recognition
 Activation
 Amplification
 Membrane Attack

15. Classical pathway
 Formation of antigen-antibody complex
 C1q binds antigen-bound antibody
 C1q activates the C1r and C1s
(a)

16. (b)
 C1s cleaves C4 and C2
 C4 binds the surface near C1 & C2
binds C4
 Formation of C4b2a (C3 convertase)

17. (c)
 C3 convertase hydolyze many C3 molecules
 C3b binds to C4b2a and forms C4b2a3b complex also called C5
convertase

18. (d)
 C4b2a3b cleave C5 into C5a and C5b

19.  C5b binds C6, initiating the formation of the membrane-attack complex
(MAC)
(e)
(f)
 Formation of poly-C9

20. Alternative Pathway
 Antibody Independent
 Involves serum proteins:C3, factor B, factor D, and properdin
 C3 cleaved into C3a and C3b
 C3b binds to foreign surface antigen
 C3b binds to factor B and form complex and stabilized by Mg2+
 C3b-boud factor B complex cleaved by factor D
 C3bBb forms and have C3 convertase activity
 C3bBb cleaves C3 and forms C3bBb3b
 C3bBb3b complex has C5 convertase activity and cleaved C5 component
into C5a and C5b

22. Lectin Pathway
 MBLectin pathway or mannan-binding lectin pathway
 Lectin are carbohydrate binding proteins
 The lectin pathway, like the alternative pathway, does not depend on
antibody for
its activation
 The mechanism is more like that of the classical pathway
 Activated by binding of mannose-binding lectin (MBL) to mannose residues
on
glycoproteins or carbohydrates on surface of microorganisms
 MBL binds to the surface of a cell or pathogen
 MBL-associated serine proteases, MASP-1 and MASP-2, bind to MBL
 The MASP-1 and -2 proteins have structural similarity to C1r and C1s and
mimic
their activities

26.  The association between an anti body and an antigen involves various
noncovalent
interactions
 The site on antigen where antibody can bind called epitope
 The interaction between antigenic determinant, or epitope, of the antigen
and the
variable-region (VH/VL) domain of the antibody molecule, particularly the
hypervariable regions, or complementarity determining regions (CDRs).
Antigen-Antibody Interactions

27.  Strength of Antigen-Antibody Interactions
 Hydrogen bonds, ionic bonds,
hydrophobic interactions, and van
der
Waals interac tions
 A strong Ag-Ab interaction
depends
on a very close fit between the
antigen and antibody

28.  The combined strength of the a single antigen-binding site on an antibody and a
single
epitope
 The association between a binding site on an antibody (Ab) with a monovalent
antigen
(Ag) can be determine
 The affinity constant, Ka, can be determined by equilibrium dialysis method
 Antibody Affinity is a quantitative measure of binding strength

29.  The strength of multiple interactions between a multivalent antibody and
antigen is called the avidity
 Precipitation Reactions
 Antibody and soluble antigen interacting in aqueous solution form a visible
precipitate
 Antibodies that aggregate soluble antigens are called precipitins
 Formation of an Ag-Ab lattice depends on the valency of both the antibody
and
antigen: The antibody must be bivalent
 The antigen must be either bivalent or polyvalent.

30. 1. Precipitation Reactions in Fluids Yield a Precipitin Curve
 A constant amount of antibody
in a series of tubes and adding
increasing amounts of antigen
to the tubes
 Each tube is centrifuged to
pellet the precipitate
 Amount of precipitate is
measured
 Plotting the amount of
precipitate
against increasing antigen
concentrations yields a
precipitin curve

31. 2. Precipitation Reactions in Gels Yield Visible Precipitin Lines
 Antigen and antibody diffuse toward one
another in agar
 Antibody is incorporated into the agar
and antigen diffuses into the antibody
containing matrix, a visible line of
precipitation will form
 Used to determine relative concentrations
of antibodies or antigens
 Two types of immunodiffusion reactions
 Radial immunodiffusion (Mancini
method)
 Double immunodiffusion
(Ouchterlony method)

32. 3. Immunoelectrophoresis Combines Electrophoresis and Double
Immunodiffusion
 The antigen mixture is first
electrophoresed to separate its components
by charge
 Antiserum is added to the troughs
 Antibody and antigen then diffuse toward
each other and produce lines of precipitation

33.  Agglutination Reactions
 The interaction between antibody and a particulate antigen results in visible
clumping called agglutination
 Antibodies that produce such reactions are called agglutinins
 An excess of antibody inhibit agglutination reactions this inhibition is
called the
prozone effect
 Prozone effect shown by high antibody concentrations, the number of
antibody
binding sites may greatly exceed the number of epitopes
 The antiserum may contain high concentrations of antibodies that bind to
the
antigen but do not induce agglutination, these antibodies, called
incomplete
antibodies

34. 1. Hemagglutination Is Used in Blood Typing
 Agglutination reactions are routinely performed to type red blood cells
(RBCs)
 In typing for the ABO antigens, RBCs are mixed on a slide with antisera to the
A
or B blood-group antigens
 If the antigen is present on the cells, they agglutinate, forming a visible
clump on the
slide
 Determination of which antigens are present on donor and recipient RBCs
 A bacterial infection often elicits the production of serum antibodies specific
for
surface antigens on the bacterial cells
 The presence of such antibodies can be detected by bacterial agglutination
reactions
2. Bacterial Agglutination Is Used To Diagnose Infection

35.  The last tube showing visible agglutination will reflect the serum antibody
titer of
the patient
 The agglutinin titer is defined as the reciprocal of the greatest serum dilution
that
elicits a positive agglutination reaction
 For example, if serial two fold dilutions of serum are prepared and if the
dilution of
1/640 shows agglutination but the dilution of 1/1280 does not, then the
agglutination titer of the patient’s serum is 640

36. 3. In Agglutination Inhibition, Absence of Agglutination Is Diagnostic of
Antigen
 Pregnancy test kits
included
latex particles coated with
human chorionic
gonadotropin (HCG) and
antibody to HCG
 A pregnant woman, which
contained HCG

37.  Agglutination inhibition assays can also be used to determine whether an indi
is using certain types of illegal drugs, such as cocaine or heroin
 An urine or blood sample is first incubated with antibody specific for the susp
drug. Then red blood cells (or other particles) coated with the drug are added.
 If the red blood cells are not agglutinated by the antibody, it indicates the sam
contained an antigen recognized by the antibody, suggesting that the individu
using the illicit drug
 One problem with these tests is that some legal drugs have chemical structure
similar to those of illicit drugs, and these legal drugs may cross-react with the
antibody, giving a false-positive reaction

38.  Detecting antigen or antibody is radioimmunoassay (RIA)
 The technique was developed in 1960 by two endocrinologists Berson and
Yalow
 The principle of RIA involves competitive binding of radiolabeled antigen
and
unlabeled antigen to a high-affinity antibody
 The labeled antigen is mixed with antibody at a concentration that
saturates the
antigen-binding sites of the antibody
 Then test samples of unlabeled antigen of unknown concentration are
added in
progressively larger amounts
Radioimmunoassay

39. As the concentration of unlabeled antigen increases, more labeled antigen
will be
displaced from the binding sites
 The decrease in the amount of radiolabeled antigen bound to specific
antibody
in the presence of the test sample is measured in order to determine the
amount
of antigen present in the test sample
 The antigen is generally labeled with a gamma-emitting isotope such as
125I, but
beta-emitting isotopes such as tritium (3H) are also routinely used as
labels.
 The antibody is covalently cross linked to Sepharose beads. The amount of
radiolabeled antigen bound to the beads can be measured after the beads
have
been centrifuged and washed
 The antibody is immobilized on the walls of microtiter wells and the
amount of
bound antigen determined.
 Separation of the Ag-Ab complex from the unbound antigen

40. Enzyme-Linked Immunosorbent Assay (ELISA) (or EIA)
 Principle- similar to RIA but depends on an enzyme rather than a radioactive
label
 An enzyme conjugated with an antibody reacts with a colorless substrate to
generate a colored reaction product
 A number of enzymes have been employed for ELISA, including alkaline
phosphatase, horseradish peroxidase, and β-galactosidase
 Allow qualitative detection or quantitative measurement of either antigen or
antibody
 A standard curve based on known concentrations of antibody or antigen is
prepared, from which the unknown concentration of a sample can be
determined

41. INDIRECT ELISA
 Antibody can be detected or quantitatively determined with an indirect ELISA
 An antigen-coated microtiter well were prepared
 Serum or some other sample containing primary antibody (Ab1) is added to
react
with the antigen attached to the well
 Any free Ab1 is washed away
 Add an enzyme-conjugated secondary anti-isotype antibody (Ab2), which
binds to
the primary antibody
 Any free Ab2 then is washed away
 A substrate for the enzyme is added
 Measure the absorbance by spectrophotometric plate readers
 Extrapolate the absorbance in standard curve (based on known
concentration) to
calculate Ab1 concentration

42.  Antigen can be detected or measured by a sandwich ELISA
 The antibody (rather than the antigen) is immobilized on a microtiter well
 A sample containing antigen is added and allowed to react with the
immobilized
antibody
 The well is washed
 A second enzyme-linked antibody specific for a different epitope on the
antigen is
added and allowed to react with the bound antigen
 Free second antibody is removed by washing, substrate is added
 The colored reaction product is measured
 Extrapolate the absorbance in standard curve (based on known
concentration) to
calculate antigen concentration
SANDWICH ELISA

43. COMPETITIVE ELISA
 Antibody is first incubated in solution with a sample containing antigen
 The antigen-antibody mixture is then added to an antigen-coated microtiter
well
 The more antigen present in the sample, the less free antibody will be
available to bind
to the antigen-coated well
 Addition of an enzyme-conjugated secondary antibody (Ab2) specific for the
isotype of
the primary antibody
 Determine the amount of primary antibody bound to the well as in an indirect
ELISA
 The higher the concentration of antigen in the original sample,the lower the
absorbance

44. CHEMILUMINESCENCE
 Measurement of light produced by chemiluminescence during certain chemical
reactions
 A luxogenic (light-generating) substrate takes the place of the chromogenic
substrate
in conventional ELISA reactions
 The advantage of chemiluminescence assays over chromogenic ones is
enhanced
sensitivity
 In general, the detection limit can be increased at least ten-fold by
switching from a
chromogenic to a luxogenic substrate

45. ELISPOT ASSAY
 Quantitative determination of the number of cells in a population that are
producing
antibodies specific for a given antigen
 The plates are coated with the antibody specific for the antigen whose
production is
being assayed
 A suspension of the cell population is then added to the coated plates
 Secreted molecules reactive with the capture molecules in the vicinity of the
secreting cells, producing a ring of antigen-antibody

46.  The plate is then washed
 Enzyme-linked antibody specific for the secreted antigen is added and
allowed to
bind
 Addition of a suitable chromogenic or chemiluminescence-producing
substrate
reveals the position of each antibody- or antigen-producing cell as a
point of color
or light
 By counting the number of colored spots, number of cytokine-secreting
cells were
present in the added cell suspension

47.  Identification of a specific protein in a complex mixture of proteins can be
accomplished by a technique known as Western blotting
 A protein mixture is electrophoretically separated on an SDS-polyacrylamide
gel
(SDS-PAGE)
 The gel is removed from the apparatus and applied to a protein-binding
sheet of
nitrocellulose or nylon
 The proteins in the gel are transferred to the sheet by the passage of an
electric current
 Addition of enzyme-linked antibodies detects the antigen of interest
 The position of the antibodies is visualized by means of an ELISA reaction that
generates a highly colored insoluble product that is deposited at the site of
the reaction
Western Blotting

49. Immunoprecipitation
 The immunoprecipitation technique allows the isolation and identification of
the
antigen of interest in a given cell for further analysis
 Procedure:
 Treatment of a cell extract containing
antigen with an antibody results in the
formation of antigen-antibody
complexes
 Addition of magnetic beads to which a
secondary antibody is linked
 It binds to the antigen-antibody
complexes
 Placing a magnet against the side of the
tube
 Allows the rapid collection of the
antigen-

50. Immunofluorescence
 Fluorescent molecules absorb light of one wavelength (excitation) and emit
light of
another wavelength (emission)
 Fluorescent compounds such as fluorescein and rhodamine are in common
use,
conjugated to the Fc region of an antibody molecule without affecting the
specificity
of the antibody Types Absorbtion Emission
Fluorescein Blue light
(490 nm)
Yellow-
green
(517 nm).
Rhodamine Yellow-green
(515 nm)
Deep red
(546 nm)
 In direct staining, the specific antibody (the primary anti-body) is
directly
conjugated with fluorescein

51.  Stained with a fluorochrome-labeled secondary reagent that binds to
the
primary antibody
 Cells are viewed under a fluorescence microscope
 In indirect staining, the primary antibody is unlabeled and is detected with
an
additional fluorochrome-labeled reagent
 Immunofluorescence has been applied to identify a number of
subpopulations of
lymphocytes, notably the CD4 and CD8 T-cell subpopulations

53. Flow Cytometry and Fluorescence
 Automate the analysis and separation of cells stained with fluorescent
antibody
 The flow cytometer uses a laser beam and light detector to count single
intact cells in
suspension
 Every time a cell passes the laser beam, light is deflected from the detector,
and this
interruption of the laser signal is recorded
 Those cells having a fluorescently tagged antibody bound to their cell
surface
antigens are excited by the laser and emit light
 That is recorded by a second detector system located at a right angle to the
laser
beam
 The instrument are capable of sorting populations of cells into different
containers
 Determination of members of a cell population bind fluorescently
labeled
antibodies is called analysis
 To place cells having different patterns of reactivity into different
containers is

54.  A mixed cell population is stained with two antibodies
 One specific for surface antigen A and the other specific for surface antigen B
 The anti-A antibodies are labeled with fluorescein (green) and the anti-B
antibodies
with rhodamine (red)
 It leaves the nozzle, each droplet receives a small electrical charge
 When a drop generated by the nozzle passes through the beam of laser light
that
excites the fluorochrome
 The intensity of the fluorescence emitted by each droplet that contains a cell
is
monitored by a detector and displayed on a computer screen
 A droplet is passing between them, it is possible to deflect the path of a
particular
droplet into one or another collecting vessel