IMMUNE CHEMISTRY

2. Immune Chemistry & Immunology
Gandham. Rajeev

3. Immunology
 Immunology deals with the study of
immunity & immune systems of vertebrates.
 Immunity (immunis = exempt/free from
burden)
 It involves the resistance shown & protection
offered by the host organism against the
infectious diseases.

4.  The immune system consists of a complex
network of cells & molecules & their
interactions.
 It is specifically designed to eliminate
infectious organisms from the body.
 This is possible since the organism is capable
of distinguishing the self from non-self, and
eliminate non-self.
 Two types-innate & adaptive or acquired.

5. Antigens
 Certain components of the cell membranes
act as specific antigens.
 They will be different from person to
person in their chemical composition and
three dimensional structure.
 The immuno-competent cells could
recognise the self from nonself.

6.  Any substance which invokes an
immunological response is an antigen or
immunogen.
 Antibody response will usually be selective
against specific spatial configurations on the
antigen, which are called antigenic
determinant sites, known as epitopes.

7. Immune response
 T - lymphocytes:
 The lymphocytes generated from the bone
marrow, passed through & processed by the
thymus gland, are then called T
lymphocytes.
 They can directly kill the target cells & are
the effector cells for the cell-mediated
immunity (CMI).

8.  The T lymphocytes are found mainly in the
paracortical areas of lymph nodes &
periarteriolar sheaths in the spleen.
 T-cells can identify viruses & microorganisms
from the antigens displayed on their surfaces.
 In peripheral blood 80% lymphocytes are T cells
& 15% are B cells.

9. Types of T-cells
 Inducer T-cells:
 Mediate the development of T-cells in the thymus.
 Cytotoxic T-cells (TC): Capable of recognizing &
killing the infected or abnormal cells.
 Helper T-cells (TH): Initiate immune responses.
 Suppressor T-cells: Mediate the suppression of
immune response.
 T-lymphocytes are effector cells for the cell-mediated
immunity (CMI)

10. B-lymphocytes
 The site of development and maturation of
B-cells occurs in bursa fabricius in birds, and
bone marrow in mammals.
 During the course of immune response, B-cells
mature into plasma cells and secrete
antibodies (immunoglobulins).
 The B cells govern the humoral immunity.

11.  The B-cells possess the capability to
specifically recognize each antigen and
produce antibodies (i.e. immunoglobulins)
against it.
 B-lymphocytes are intimately associated
with humoral immunity.

12. Immunoglobulins
 lmmunoglobulins, a specialised group of proteins.
 Associated with γ-globulin fraction (on
electrophoresis) of plasma proteins.
 Some immunoglobulins, separate along with β & α-
globulins.
 So γ-globulin & immunoglobulin are not
synonymous.
 lmmunoglobulin is a functional term while γ-globulin
is a physical term.

13. Structure of immunoglobulins
 All the immunoglobulin (Ig) molecules
basically consist of 2 identical heavy (H)
chains (mol. wt. 53,000 to 75,000 each) & 2
identical light (L) chains (mol. wt. 23,000 each)
 Held together by disulfide linkages & non-covalent
interactions.
 Immunoglobulin is a Y-shaped tetramer
(H2L2).

14.  Each heavy chain contains approximately
450 amino acids
 Each light chain contains approximately 212
amino acids.
 The heavy chains of Ig are linked to
carbohydrates, hence immunoglobulins are
glycoproteins.

16. Constant and variable regions
 Each chain (L or H) of lg has two regions
(domains),
 Constant & variable.
 The amino terminal half of the light chain is
the variable region (VL) while the carboxy
terminal half is the constant region (CL).

17.  Heavy chain, approximately one-quarter of
the amino terminal region is variable (VH)
while the remaining three quarters is
constant (CH1,, CH2, CH3).
 The amino acid sequence (with its tertiary
structure) of variable regions of light & heavy
chains is responsible for the specific binding
of immunoglobulin (antibody) with antigen.

18. Fab and Fc Portions
 Papain (proteolytic enzyme from papaya)
cleaves the lg.
 Two Fab (fraction antibody) portions & one
Fc (fraction crystallizable) portion are
produced.
 The antigen binding part of the antibody is
in the Fab fragment.

19.  The cleavage takes place in the hinge
region, where lg molecule can have
mobility in 3 dimensional space, so as to
adjust for tight grip on the antigen.
 Carbohydrate groups of the lg molecule are
also situated in the hinge region.
 The area capable of complement binding
lies in the Fc portion.

20.  Pepsin cleaves lg at another site so as to
yield F(ab)2, where 2 Fab portions are
combined together.
 Fab part can combine with antigen very
weakly, but combination with F(ab)2 is
stronger.

22. Classes of Immunoglobulins
 Immunoglobulin-G (lgG) is made up of
heavy chain γ (gamma)
 lgM has μ (mu) heavy chain
 lgA has α (alpha) heavy chain
 lgD contains δ (delta)
 lgE heavy chain is called ε (epsilon).

23.  The light chains are two types either κ
(kappa) or λ (lambda) in all the classes.
 An Ig (of any class) contains 2κ or 2λ light
chains & never a mixture.
 E.g: lgG may consist of either γ2 κ2 or γ2 λ2.
 In human beings, 60% light chains are of κ
variety and 40% are of λ type.

24. Immunoglobulin G (IgG)
 lgG contains 2 heavy chains & 2 light chains;
 Heavy chains being of gamma type.
 IgG is the most abundant (75-80%) class of
immunoglobulins.
 IgG is composed of a single Y-shaped unit
(monomer).
 It can pass from vascular compartment to
interstitial space.

25.  It can cross-placental barrier & protects the
newborn child from infections.
 IgG is the only immunoglobulin that can
cross the placenta and transfer the
mother's immunity to the developing fetus.
 IgG triggers foreign cell destruction
mediated by complement system.

26. Immunoglobulin A (IgA)
 IgA usually are dimers (total 4 heavy chains
and 4 light chains).
 The J chain connects the dimers.
 They are the secretory antibodies seen in
secretions of gastrointestinal tract,
nasopharyngeal tract, urogenital tract, tears,
saliva, sweat, milk, etc.

27.  Ig A is the most predominant antibody in
the colostrum, the initial secretion from the
mother's breast after a baby is born.
 The IgA molecules bind with bacterial
antigens present on the body (outer
epithelial) surfaces & remove them.
 IgA prevents the foreign substances from
entering the body cells.

28. Immunoglobulin M (IgM)
 lgM is the largest immunoglobulin
composed of 5 Y-shaped units
 IgM is a pentamer.
 Five subunits, each having 4 peptide chains
(total 10 heavy chains & 10 light chains) are
joined together by a J-chain polypeptide.

29.  It can combine with 5 antigenic sites due to
its pentameric structure.
 Due to its large size, IgM cannot traverse
blood vessels, restricted to the blood
stream.
 IgM is the first antibody to be produced in
response to an antigen & is the most
effective against invading microorganisms

30. Immunoglobulin D (IgD)
 IgD is composed of a single Y-shaped unit
& is present in a low concentration in the
circulation.
 IgD molecules are present on the surface
of B cells.
 IgD may function as B-cell receptor.

31. lmmunoglobulin E (IgE)
 IgE is a singleY- shaped monomer.
 It is normally present in minute concentration
in blood.
 IgE levels are elevated in allergic conditions
as it is associated with the body's allergic
response.
 IgE molecules tightly bind with mast cells
which release histamine & cause allergy.

32.  IgG, IgE and IgD have one basic unit each.
 Ig M has 5 basic units and IgA has 2 basic units.
Red circles represent J pieces.
 Green squares are secretory pieces

33. Production of Igs by multiple genes
 Igs are composed of light & heavy chains.
 Each light chain is produced by 3 separate
genes,
 Variable region (VL) gene.
 Constant region (CL) gene
 Joining region (J) gene.

34.  Each heavy chain is produced by at least 4
different genes
 Variable region (VH) gene
 Constant region (CH) gene
 Joining region (J) gene
 Diversity region (D) gene.
 Multiple genes are responsible for the
synthesis of any one of the immunoglobulin.

35. Multiple myeloma
 Multiple myeloma, a plasma cell cancer,
constitutes about 1% of all cancers affecting
the population.
 Females are more susceptible
 It usually occurs in the age group 45-60 years.

36. Abnormal lg production
 Multiple myeloma is due to the
malignancy of a single clone of plasma
cells in the bone marrow.
 This results in the overproduction of
abnormal immunoglobulins, mostly (75%)
IgG & in some cases (25%) IgA or IgM.

37.  IgD type multiple myeloma found in
younger adults is less common (<2%) but
more sever.
 In patients of multiple myeloma, the
synthesis of normal immunoglobulins is
diminished causing depressed immunity.
 Hence recurrent infections are common in
these patients.

38. Electrophoretic pattern
 There is a sharp & distinct band (M band,
for myeloma globulin) between β- & γ-
globulins.
 This M band almost replaces the γ -globulin
band due to the diminished synthesis of
normal γ –globulins.

39. Bence Jones proteins
 These are the light chains (κ or λ) of Igs that
are synthesized in excess.
 Bence Jones proteins have a molecular
weight of 20,000 or 40,000 (for dimer).
 About 20% of the patients of multiple
myeloma, Bence jones proteins are
excreted in urine which often damages the
renal tubules.

40. Amyloidosis
 Amyloidosis is characterized by the
deposits of light chain fragments in the
tissue (liver, kidney, intestine) of multiple
myeloma patients.

41. Detection of Bence Jones Proteins in urine
 Electrophoresis of a concentrated urine is the
best test to detect Bence Jones proteins in
urine.
 Bradshaw's test:
 It involves layering of urine on concentrated
HCI that forms a white ring of precipitate, if
Bence Jones proteins are present

42.  The classical heat test involves the
precipitation of Bence Jones proteins when
slightly acidified urine is heated to 40-50°C.
 This precipitate redissolves on further
heating of urine to boiling point.
 It reappears again on cooling urine to about
70°C

43. References
 Text book of Biochemistry – DM Vasudevan
 Text book of Biochemistry – U Satyanarayana
 Text book of Biochemistry – MN Chatterjea

44. Thank you