1. Antigen-antibody reactions occur when antigens and antibodies come into contact, such as when foreign substances enter the body. 2. These reactions are characterized by non-covalent bonds between the antigen and antibody, including hydrogen bonds, electrostatic bonds, hydrophobic bonds, and Van der Waals forces. 3. Consequences of antigen-antibody reactions include opsonization, activation of immune cells through Fc receptors, complement activation, and neutralization of pathogens or toxins.

2. 1. Definition of Antigen-Antibody Reactions
2. Antibodies
3. Antigens
4. Nature of Antigen- Antibody reactions
5. Characteristics of antigen-antibody
reactions
6. Types of antigen-antibody reactions
7. Consequences of antigen-antibody
reactions

3.  It is the reaction between an antigen and
an antibody when they come in contact.
 This occurs when antigens enter the body.
 When antigens enter the body, signals are
sent out and in response to that antibodies
are made specifically to destroy the
antigens.
 It is part of the Adaptive(Specific) immune
system.

4.  Antibodies are also known as Immunoglobulin (Ig)
 These are globulin proteins that are found in the
blood or other bodily fluids of vertebrates.
 Antibodies are used by the immune system to identify
and neutralize foreign objects such as bacteria and
viruses.
 Antibodies are produced by the Plasma Cells and
are the receptors of the B- cells.
 An Antibody is typically a Y-shaped structure
consisting of four polypeptide chains- two Heavy
chains and two Light chains. Each arm of the „Y‟
bears an antigen binding site.

7.  An antigen is any substance that triggers
your immune system to produce
antibodies against it.
 It may be a foreign substance from the
environment such as chemicals,
bacteria, viruses, or pollen.
 It may also be formed within the body,
as with bacterial toxins or tissue cells.
 Antigens have different structures and
types.

8.  Antigen is a substance that reacts with
the products of a specific immune
response( specific antibodies)
 Immunogen is a substance that induces
a specific immune response

9. 1. Lock and key concept
- Fab ( antigen- binding fragment) is the
combining site which is constructed by
Hypervariable regions( H1-H4) of the heavy
and light chains that interacts with an
antigenic determinant(epitope) which forms a
lock and key concept where lock= antibody
and key= antigen
- X-ray crystallographers studies of Ag-Ab
interactions shows that antigenic determinant
is nestled in a cleft formed by the combining
sites of the antibody as illustrated…

11. 2. Non- Covalent Bonds
 There are Four invisible forces that enables the binding of
the antigen and the antibody and these are:
a. Hydrogen Bond
- When the antigen and antibody are in very close
proximity, relatively weak hydrogen bonds can be formed
between hydrophilic groups (e.g., OH and C=O, NH and
C=O, and NH and OH groups).
b. Electrostatic Bond
- Electrostatic bonds result from the attraction between
oppositely charged ionic groups of two protein side
chains, for example, an ionized amino group (NH4) on a
lysine in the antibody and an ionized carboxyl
group(COO) on an aspartate residue in the antigen.

12. c. Hydrophobic Bond
- the attractive force between molecules due to the close
positioning of non- hydrophilic portions of the two molecules ( ie
antigen and antibody)
d. Van der Waal’s forces
- These forces depend upon interactions between the “electron
clouds” that surround the antigen and antibody molecules. The
interaction has been compared to that which might exist
between alternating dipoles in two molecules, alternating in
such a way that at any given moment oppositely oriented
dipoles will be present in closely apposed areas of the antigen
and antibody molecules.
3. Reversibility
- it is reversible since the Ag- Ab rxns are via non- covalent
bonds.

13.  ANTIBODY AFFINITY
 Strength or force of attraction of a rxn
btwn a single antigenic determinant
(epitope) at a single combining site on
the Ab.
 ↑ the affinity of Ab for Ag, the more
stable the interaction will be.

14.  ANTIBODY AVIDITY
 Overall strength of binding of several
antibodies produced in response to an
immunogen, which presents several
different epitopes to the immune system.
 ↑ the avidity, the more stable the Ag-Ab
reaction

15. Fig. 8.3 Diagrammatic representation of the
avidity concept. The binding of antigen
molecules by several antibodies of different
specificities (A) stabilizes the immune
complex, since it is highly unlikely that all
AgAb reactions dissociate simultaneously at
any given point of time (B). (Redrawn from
Roitt, I. Essential Immunology, 4th ed.
Blackwell, Oxford, 1980.)

16.  ANTIBODY SPECIFICITY
 refers to the ability of an individual
antibody combining site to react with
only one epitope or the ability of a
population of Ab molecules to react with
only one antigen.
 In general, there is a high degree of
specificity in Ag-Ab rxns.

17.  CROSS REACTIVITY
 refers to the ability of an individual
antibody combining site to react with
more than one epitopes.
 It arises because the cross reacting
antigen shares an epitope in common
with the immunizing antigen or becoz it
has an epitope which is structurally
similar to one of the immunizing antigen.

18. Fig. 8.4 Diagrammatic representation
the concept of cross-reaction betwe
complex antigens.
An antiserum containing several
antibody populations to the
determinants of a given antigen will
react with other antigens sharing
common or closely related
determinants. The avidity of the
reaction will decrease with decreasi
structural closeness, until it will no
longer be detectable. The reactivity
of the same antiserum with several
related antigens is designated as cro
reaction. (Redrawn from Roitt, I.
Essential Immunology, 4th ed.
Blackwell, Oxford, 1980.)

19.  AGGLUTINATION REACTIONS
- the aggregation by antibody of already
insoluble particles or cells, e.g., bacteria
or red cells, into larger clumps.

20. Fig. 8.8 IgM antibodies are more
efficient in inducing red cell
agglutination. Red cells remain at
the same distance from each
other due to their identical
electrical charge (zeta potential).
IgG antibodies are not large
enough to bridge the space
between two red cells, but IgM
antibodies, due to their
polymeric nature and size, can
induce red blood cell
agglutination with considerable
ease.

21.  PRECIPITATION REACTIONS
A. Quantitative Reaction:
- adding increasing amount of antigen
to a known amount of antibodies results in
a progressive increase in the amount of
specific precipitate, which reaches a
maximum level and then declines.
- precipitation occurs when there is more
binding and cross linking take place and it‟s
at its highest peak when there are no free-
antibodies and free- antigens in the
solution.

23. B. Qualitative Reactions
- various procedures are used to
determine the presence, identity, and
specificity of soluble antigens and
antibodies.
- Procedures include:
- Ring precipitin tests, agar diffusion
tests, and immunoelectrophoresis are
commonly done in the laboratory.

24. Fig. 8.7 Diagrammatic representation of
a reaction of double immunodiffusion.
Antigen and antibody are placed in
opposite wells carved in a semi-solid
medium (e.g., agarose gel). Both
antigen and antibody diffuse in all
directions and towards each other,
reacting and eventually reaching
equivalence, at which point a linear
precipitate appears between the
antigen and antibody wells.

25.  Opsonization
Antigen- Antibody rnx
↓
Antibodies unfold
↓
interаcts with Fc receptor
on phаgocytic cells
↓
phаgocytosis

26.  Fc – receptor mediated cell activation
ag-ab complexes + phagocytic cells through their Fc
receptors
↓
activates ingesting cell (phagocytes)→ inflammatory
reaction (less favorable outcome)
↓
enhancement of its microbicidal activity.
NB: Another adverse reaction results from the
engagement of Fc receptor–bound IgE on basophils and
mast cells with their corresponding antigen. The result of
this reaction is the release of the potent mediators that
trigger an allergic reaction .

27.  Complement Activation
 Complement activation is an extremely
important amplification mechanism, which
mediates antibody-dependent
neutralization and elimination of infectious
agents. There аre two mechаnisms
involved:
1. Generation of C3b →deposited on the
membrane of a microorganism →facilitates
phagocytosis by cells with C3b receptors.

28. 2. Disruption of lipid bilayers that depends
on the generation of the late
complement components (C6–C9).
late complement components (C6–C9)
→ properly assembled on a cell
membrane → induce formation of
transmembrane channels → cell lysis.

29.  Neutrаlisаtion
 аntibodies bind to bаcteriа, viruses, or
toxin (Ag- Ab rnx) → prevents the
interaction of the microbial agents or
their products with the receptors that
mediate their infectiveness or toxic
effects → infectious agent or the toxin
become harmless( neutralized.)

30.  Antigen- Antibody reactions are interactions
between antigens and antibodies when they come
into contact.
 Nature of the Ag-Ab reactions are as follows: Lock
and Key Concept; Non- covalent bonds (Hydrogen,
Electrostatic, and Hydrophobic bonds, and Van der
Waal‟s Forces; and Reversibility.
 Characteristics of Ag- Ab reactions are: Affinity;
Avidity; Specificity; and Cross- reactivity.
 The Specific Types of Ag- Ab reactions include:
Agglutination and Precipitation Reactions
 Consequences of the Ag-Ab reactions are:
Opsonization; Fc- receptor mediator Cell activation;
Complement Activation; and Neutralization.

31. CAI, Color Atlas of Immunology[PDF]. Burmester G,
Pezzutto A[MD]. Thieme Stuttgart, New York;2003
Google Browser[ database on the Internet]. Mayer. G
[MD]: Southern Carolina University School of Medicine
(US); 2010. Available from:
http://pathmicro.med.sc.edu/mayer/ab-agrx.html
Hildamann W.H. Essentials of Immunology. New York,
Elsevier Science Publishers; 1984
MI, Medical Immunology[PDF]. Virella G, ed. 5th Ed.
New York, Marcel Dekker, Inc. 2001

32. Figure 1
In an antigen, the same antigenic
determinant repeated many times
Figure 3
Figure 2 Hapten-carrier conjugates have native
T-dependent antigens are antigenic determinants of the carrier as
characterized by a few copies of many well as new determinants of the
different antigenic determinants hapten

33. Figure 4
Antigenic determinants are usually Figure 5
limited to those portions of the antigen Superantigens activate a large
that are accessible to antibodies fraction of T cells in contrast to
shown in black for this iron-containing conventional T-dependent
protein antigens