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1. Dr. M. Krishna Vasudev Immunoglobulins: Structure and
Function
2. Immunoglobulin Immunoglobulin is a glycoprotein that is made
in response to an antigen and can recognize and bind to the antigen
that caused its production.
3. Are gamma globulins Synthesized by plasma cells Constitute
25-30 % of total serum proteins Antibodies are present in serum,
tissue fluids and mucosal surfaces. All antibodies are
immunoglobulins, but all immunoglobulins may not be antibodies
4. Basic structure Composed of 4 polypeptide chains. 2
identical light and 2 identical heavy chains Linked by disulphide
bonds Light chains similar in all immunoglobulins Light chains
occur in 2 varieties kappa and lambda Light and Heavy chains are
subdivided into variable and constant region. Each heavy and light
chain contains amino terminal in variable region carboxy terminal
in constant region
5. Heavy chains are structurally and antigenically distinct for
each class Each immunoglobulin peptide chain has intra chain
disulphide bonds- form loops Each loop is compactly folded to form
a globular structure- domain Light chain contains a single variable
domain (VL) and a single constant domain (CL). Heavy chain contains
one variable domain (VH) and 3 constant domains (CH1, CH2, CH3)
Hinge region is the segment in heavy chain - between CH1, CH2
6. Digestion with proteolytic enzymes Papain enzyme Peptide
bonds in the hinge region are broken Produces 3 fragments 2
identical fragments called Fab fragments antigen binding activity.
Other fragment called Fc fragment (Fraction crystallizable)
7. Pepsin digestion Produce a single fragment composed of two
Fab like subunits F(ab)2 binds antigen Fc fragment is not
recovered- digested to small numerous peptides.
8. Classification Based on structure and antigenic nature of H
chain the immunoglobulins are classified into 5 classes. Ig G-
(gamma) Ig A- (alpha) Ig M- (mu) Ig D- (delta) Ig E -
(epsilon)
9. Immunoglobulin G (Ig G) Most abundant class in serum
Constitutes 80% total immunoglobulin Present in blood, plasma and
tissue fluids Contains less carbohydrate than other immunoglobulins
It has a half life of 23 days: the longest of all of the
immunoglobulin isotypes
10. Crosses placenta and provide natural immunity to foetus and
neonate at birth Acts against bacteria and viruses by opsonizing
Neutralize toxin Activate complement by classical pathway
Catabolism of IgG is unique in that it varies with its serum
concentration
11. Sub classes of Ig G Ig G1, Ig G2, Ig G3, Ig G4.
12. Biological function of subclasses IgG1, IgG3, IgG4 cross
placenta and protect foetus IgG3 activates complement IgG1 and IgG3
binds to Fc receptor on phagocytic cells, monocytes and macrophages
and mediate opsinization.
13. Immunoglobulin A (Ig A) Constitutes 10-15 % of total
immunoglobulins Present in milk, saliva, tears, mucous of
respiratory tract, digestive tract and genitourinary tract. In
serum exist as monomer In external secretions exist as dimer called
secretory Immunoglobulin. Has J chain and secretory piece. Half
life: 6-8 days
14. Formation of secretory Ig A Dimeric Ig A binds to the
receptor on the surface of the epithelial cells - endocytosed and
transported across the cell to the luminal surface After reaching
the surface, the poly-Ig receptor is cleaved The portion of the
receptor that remains attached to the Ig A dimer secretory
component Secretory piece protects Ig A from digestive enzymes and
denaturation by
15. Functions Provides local immunity. Secretory Ig A binds to
surface antigens of microorganism and prevent its attachment and
invasion of the mucosal surfaces of respiratory and digestive
tract- immune elimination. Secretory IgA provides important line of
defense against salmonella, Vibrio cholerae, N. gonorrhoeae,
influenza virus and poliovirus. Secretory IgA present in breast
milk protects newborn during first months of life. Activates
complement by the alternative pathway Promotes phagocytosis and
intracellular killing of microorganisms
16. Immunoglobulin M (Ig M) Accounts for 5-10% of total serum
proteins Polymer of five monomeric units (pentamer) Held together
by disulfide bonds and J chain Mol. Wt. of 900,000- 10,00,000
(millionaire molecule) Half life: 5 days
17. Most of IgM (80%) present intravascularly Present in low
concentration in intercellular tissue fluids Cannot cross placenta
Presence of IgM antibody in serum of newborn indicate congenital
infection. Earliest immunoglobulin to be synthesized by foetus (20
weeks) First immunoglobulin to be produced in primary response to
antigen Relatively short-lived hence its demonstration in the serum
indicates recent infection Monomeric IgM appears on the surface of
unstimulated B lymphocytes and act as receptors
18. Functions It agglutinates bacteria Activates complement by
classical pathway Causes opsonization and immune heamolysis
Believed to be responsible for protection against blood invasion by
microorganisms
19. Immunoglobulin E (Ig E) Structure is similar to Ig G Has 4
constant region domains. Mol. Wt. 1,90,000 Half life: 2 days Heat
labile (inactivated at 560C in 1 hour) Normal serum concentration
0.3 ug/ml Mostly present extra cellularly Does not cross
placenta
20. Produced in the lining of respiratory and intestinal tract
Known as reagin antibody Does not activate complement nor
agglutinate antigens Binds to the Fc receptors on the membranes of
blood basophils and tissue mast cells Mediates immediate
hypersensitivity reaction and P.K. reaction Responsible for
symptoms of anaphylactic shock, hay fever and asthma. Play a role
in immunity against helminthic parasites
21. IgE binds to Fc receptors on the membrane of blood
basophils and tissue mast cells. When two IgE molecules on the
surface of these cells are cross linked by binding of the same
antigen- cells degranulates. Release histamine and pharmacological
mediators of anaphylaxis from cell. The physiological role of IgE
appears to be protection against pathogens by mast cell
degranulation and release of inflammatory mediators
22. Mediates P.K. reaction (PRAUSNITZ & KUSTNER) The
presence of a serum component responsible for allergic reaction was
first demonstrated by Prausnitz and Kustner in 1921. Kustner was
suffering from atopic hypersensitivity to certain species of fish
Kustners serum was injected intracutaneously in Prausnitz After 24
hrs small quantity of cooked fish antigen was injected at the same
site A wheal and flare reaction occurred within minutes.
23. Immunoglobulin D (Ig D) Structure is similar to IgG Serum
concentration 30 micrograms per ml Constitutes 0.2% of total
immunoglobulins Half life: 3 days IgD together with IgM is major
membrane bound immunoglobulin on unstimulated B lymphocytes-acts as
recognition receptors for antigens
24. Properties and biological activities of Immunoglobulins Ig
G Ig A Ig M Ig D Ig E 1. Structure Monomer Monomer in serum Dimer
in secretion Pentamer Monomer Monomer 2. Heavy chain CH domain
Gamma Three Alfa Three Mu Four Delta Three Epsilon Four 3. Mol. Wt.
1,50,000 1,60,000 9,00,000 1,80,000 1,90,000 4. Serum concentration
(mg/ml) 12 2 1.2 0.03 0.00004 5. Present on membrane of mature B
cell _ _ + + _ 5. Intravascular Distribution (%) 45 42 80 75 50 6.
Crosses placenta + - - - - 7. Present in milk + + - - - 8.
Selective secretion by seromucous glands - + - - - 9. Activation of
complement Classical Alternate + - - + + - - - - - 10 Binds to FC
receptor of phagocytes + - - - - 11 Induces mast cell degranulation
- - - - +
25. Complement System
26. Consists of approx. 20 proteins that are present in normal
human serum synthesized mainly by liver Heat-labile inactivated by
heating serum at 560C for 30 minutes Able to augment the effects of
other components of the immune system Important component of innate
host defenses
27. Three main effects: 1. Lysis of cells (bacteria,
allografts, tumor cells) 2. Generation of mediators of inflammation
3. Opsonization enhancement of phagocytosis
28. Sequential activation of complement components occurs via
one of three pathways: 1. Classic pathway 2. Lectin pathway 3.
Alternative or Properdin pathway Lectin and alternative pathways
are more important the first time we are infected by microorganisms
because antibody required to activate the classic pathway is not
yet present
29. Part of acquired or adaptive immunity Activated by Ag-Ab
complexes Immunoglobulins involved: IgM and IgG (except IgG4)
Involves activation of C1 Composed of C1q, C1r, and C1s binds to Fc
portion of IgG and IgM Requires calcium for activation Classic
Pathway
30. Other activators include: 1. Viruses Murine and
Retroviruses 2. Bacteria Mycoplasma 3. Polyanions, especially bound
to cations a. PO4 3- - DNA, lipid A, cardiolipin b. SO4 2- -
dextran, heparin, chondroitin 4. Arrays of terminal mannan groups
Classic Pathway
31. Classic Pathway Components of the Classical Pathway Native
component Active component(s) Function(s) C1(q,r,s) C1q Binds to
antibody that has bound antigen, activates C1r. C1r Cleaves C1s to
activate protease function. C1s Cleaves C2 and C4. C2 C2a Unknown.
C2b Active enzyme of classical pathway; cleaves C3 and C5. C3 C3a
Mediates inflammation; anaphylatoxin. C3b Binds C5 for cleavage by
C2b. Binds cell surfaces for opsonization and activation of
alternate pathway. C4 C4a Mediates inflammation. C4b Binds C2 for
cleavage by C1s. Binds cell surfaces for opsonization.
32. Classic Pathway Components of the Membrane-Attack Complex
Native component Active component(s) Function(s) C5 C5a Mediates
inflammation; anaphylatoxin, chemotaxin. C5b Initiates assembly of
the membrane-attack complex (MAC). C6 C6 Binds C5b, forms acceptor
for C7. C7 C7 Binds C5b6, inserts into membrane, forms acceptor for
C8. C8 C8 Binds C5b67, initiates C9 polymerization. C9 C9n
Polymerizes around C5b678 to form channel that causes cell
lysis.
33. Classic Pathway
34. Alternative Pathway Also known as the Properdin Pathway
Part of innate immunity Bypasses C1, C4, and C2 Does not require an
antigen-binding protein Does not wait for antibody to be formed for
activation Acts synergistically with the classical pathway
35. Alternative Pathway Usually activated by products of
micro-organisms like endotoxin Other activators include: 1.
Complexes containing IgA 2. Some virus-infected cells (e.g. EBV) 3.
Many gram negative and gram positive organisms 4. Parasites
Trypanosomes, Leishmania 5. Dextran SO4 6. Erythrocytes 7.
Carbohydrates (agarose)
36. Alternative Pathway Components of the Alternate Pathway
Native component Active component(s) Function(s) C3 C3a Mediates
inflammation; anaphylatoxin. C3b Binds cell surfaces for
opsonization and activation of alternate pathway. Factor B B Binds
membrane bound C3b. Cleaved by Factor D. Ba Unknown. Bb Cleaved
form stabilized by P produces C3 convertase. Factor D D Cleaves
Factor B when bound to C3b. Properdin P Binds and stabilizes
membrane bound C3bBb.
37. Alternative Pathway
38. Lectin Pathway Also known as the MBL Pathway Activated by
binding of mannose-binding lectin (or mannose-binding protein) to
surface of microbes bearing mannan (polymer of the sugar mannose)
in a calcium dependent manner Binding causes activation of MASP
(MBP- associated serine proteases) cleave C2 and C4
39. Lectin Pathway
40. All three pathways lead to production of C3b central
molecule of complement cascade Presence of C3b on surface of a
microbe marks it as foreign and targets it for destruction C3b with
two important functions: 1. Combines with other complement
components to generate C5 convertase 2. Opsonizes bacteria
41. Biologic Effects: 1. Opsonization C3b & C1q; enhance
phagocytosis 2. Chemotaxis C5a and C5,6,7 complex attract
neutrophils C5a enhance adhesiveness of neutrophils to the
endothelium 3. Anaphylatoxin (C3a, C4a, C5a) Cause degranulation of
mast cells Bind directly to smooth muscles of bronchioles
bronchospasm
42. Biologic Effects: 4. Cytolysis (MAC) Disrupt the membrane
& the entry of water and electrolytes into the cell 5.
Enhancement of antibody production Binding of C3b to its receptors
on the surface of activated B cells enhanced antibody
production
43. Regulation of Complement System 1. C1 inhibitor (C1-INH)
Important regulator of classic pathway A serine protease inhibitor
(serpin) Irreversibly binds to and inactivates C1r and C1s, as well
as MASP in lectin pathway 2. Factor H Regulate alternative pathway
Reduce amount of C5 convertase available With both cofactor
activity for the factor I- mediated C3b cleavage, and decay
accelerating activity against C3bBb (C3 convertase)
44. Regulation of Complement System 3. Properdin Protects C3b
and stabilizes C3 convertase 4. Factor I Cleaves cell-bound or
fluid phase C3b and C4b inactivates C3b and C4b 5. Decay
accelerating factor (DAF) Glycoprotein on surface of human cells
Prevents assembly of C3bBb or accelerates disassembly of preformed
convertase no formation of MAC Acts on both classical and
alternative
45. Regulation of Complement System 6. C4b-binding protein
(C4BP) Inhibits the action of C4b in classical pathway Splits C4
convertase and is a cofactor for factor I 7. Complement Receptor 1
(CR-1) Co-factor for factor I, together with CD46 8. Protectin
(CD59) and Vitronectin (S protein) Inhibits formation of MAC by
binding C5b678 Present on self cells to prevent complement from
damaging them
46. Clinical Aspects 1. Deficiency of C5-C8 &
Mannan-binding lectin Predispose to severe Neisseria bacteremia 2.
Deficiency of C3 Severe, recurrent pyogenic sinus & resp. tract
infections 3. Deficiency of C1 esterase inhibitor Angioedema inc.
capillary permeability and edema 4. Deficiency of DAF Increased
complement-mediated hemolysis paroxysmal nocturnal
hemoglobinuria
47. Clinical Aspects 5. Transfusion mismatches Activation of
complement generate large amounts of anaphylatoxins & MAC red
cell hemolysis 6. Autoimmune diseases Immune complexes bind
complement low complement levels + activate inflammation tissue
damage 7. Severe liver disease Deficient complement proteins
predispose to infection with pyogenic bacteria
48. Clinical Aspects 8. Factor I deficiency Low levels of C3 in
plasma due to unregulated activation of alternative pathway
recurrent bacterial infections in children Mutations in factor I
gene implicated in development of Hemolytic Uremic Syndrome
49. Dr. M. Krishna Vasudev Dept of General Medicine
Immunodeficiency disorders
51. Mechanisms of Immunodeficiency Loss or reduction of: Cell
type Cell numbers Cell function
52. Loss of Cell Function Receptors Cell signaling Cytokine
production Ig production Co stimulation impairment Intracellular
killing Extravasation impairment
53. Primary Immunodeficiency Myeloid lineage Congenital
agranulocytosis Leukocyte-adhesion deficiency Lymphoid lineage
Severe combined immunodeficiency (SCID) B cells Agammaglobulinemia
Hypogammaglobulinemia Specific Ig Deficiencies T cells DiGeorge
Syndrome Wiskott Aldrich Syndrome
54. Secondary Immunodeficiency Drug related Disease related
Cancer AIDS HIV T helper cell as target
55. SCID Various genetic defects No TCR or defective TCR
Defective cell signaling Defective IL 2 Recurrent infections Death
at early age
56. Wiskott Aldrich Syndrome X linked disorder Affects platelet
numbers/function Affects T cell function Cytoskeleton of
lymphocytes affected Lower amounts of IgM Increased susceptibility
to certain bacterial infections
57. Brutons Agammaglobulinemia Low levels of IgG B cell signal
transduction affected Defective BCR Recurrent bacterial diseases
starting at end of first year of life Short life span
58. DiGeorge Syndrome Poorly developed or functioning thymus
Associated with other developmental conditions Depression of T cell
numbers Absence of T cell response Humoral response to T
independent antigens only
59. Secondary Immunodeficiency Drugs Irradiation Cancer
AIDS
60. AIDS HIV as agent T helper as target cell Attacked by own
CD8 cells Susceptible to opportunistic infections Kaposis sarcoma
CMV Candida Pneumocystis carinii