Download antibody - Yengage

ANTIBODY
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ANTIBODY
• Humoral basis of immunity
• Secreted by plasma cell
• Present on B cell membrane
• React with antigen - specifically observable
• Antibody - phagocytosis, phagocytosis,
ADCC
• Sera - high antibody levels - ‘immune sera’
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ANTIBODY
• Fractionation of immune sera - soluble
albumin, insoluble globulin
• Globulins - water soluble pseudoglobulin,
insoluble globulin
• Most antibodies - globulins
• Tiselius (1937) - serum proteins - albumin,
alpha, beta, gamma globulin
• Antibody activity – gamma globulin fraction
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ANTIBODY
Serum electrophoresis showing γ globulin nature of antibodies
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ANTIBODY
• 1964 – WHO - immunoglobulin
• Protein - known antibody activity and other
proteins related to them by chemical structure
• Includes - abnormal proteins – myeloma
• Macroglobulinemia
• Cryoglobulinemia
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ANTIBODY
• Immunoglobulins - synthesised by plasma
cells and by lymphocytes
• Immunoglobulins - structural and chemical
concept
• Antibody - functional and biological
concept
• All antibodies - immunoglobulins
• All immunoglobulins - may not be
antibodies
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ANTIBODY - STRUCTURE
ENZYME DIGESTION
Rabbit lgG antibody - to egg albumin
digested by papain
split - two fractions
Insoluble fraction
Soluble fraction
Crystallised in cold
Bound with egg albumin
Fc (crystallisable)
Fab (antigen binding)
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ANTIBODY STRUCTURE
Each molecule of immunoglobulin
Split by papain - three parts
One Fc - Two Fab
When treated with pepsin
Two Fab fragments - held together
Bivalent - F(ab)2
Fc portion – digested - small fragments
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ANTIBODY STRUCTURE
Basic structure of an immunoglobulin molecule and the fragments obtained
by cleavage by papain and pepsin
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IMMUNOGLOBULIN CHAINS
• Each molecule - 4 polypeptide chains
• Smaller chain - Light (L) chain
• Longer chain - Heavy (H) chain
• ‘L’ chain attached to ‘H’ chain by disulphide
bond
• Two ‘H’ chains - joined together by 1-5 S-S
bond
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IMMUNOGLOBULIN CHAINS
H chain-structurally, antigenically distinct
Designated by Greek letter
Immunoglobulin class
H chain
lg G
 (gamma)
lg A
α (alpha)
lg M
 (mu)
lg D
 (delta)
lg E
 (epsilon)
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IMMUNOGLOBULIN CHAINS
• L chains - similar in all classes of
immunoglobulins
• Two varieties - kappa (K), lambda ()
• A molecule of immunoglobulin may have
either kappa or lambda, never both
• Kappa andlambda named after Korngold
and Lapari
• Each light chain is bound to a heavy chain
by interchain and intrachain disulphide
bonds
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IMMUNOGLOBULIN CLASSES
• Antigen combining site - at amino terminus
- composed of both ‘H’ and ‘L’ chains
• Amino terminal - amino acid sequencevariable – variable region - antigen
recognition
• Carboxy terminal - constant regioneffector functions
• Infinite range - antibody specificity
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IMMUNOGLOBULIN
Structure of immunoglobulin molecule
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HYPERVARIABLE REGION
• Amino acid sequence of variable region of L
and H chains
• Consist of relatively invariable and highly
variable zones
• Highly variable - three in L four in H
• Hypervariable - ‘hot spots’ sites on
hypervariable regions that make contact
with epitope ‘complementary determining
region’ or CDR
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IMMUNOGLOBULIN CLASSES
• IgG
• IgA
• IgM
• IgD
• IgE
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IgG
• Major serum immunoglobulin 80% of total
• Mol wt - 150,000(7S)
• Half-life - 23 days
• Level raised - chronic malaria, kala azar,
myeloma
• lgG - transported across placenta
• Natural passive immunity - newborn
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IgG
• lgG - binds microorganism - enhances their
phagocytosis
• Extracellular killing - target cell - coated
with lgG antibody
• Mediated through recognition of surface of
Fc fragment
• lgG complex - platelet Fc receptor
• Aggregation of vasoactive amines
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IgG
Structure of immunoglobulin molecule
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IgG
• Participates in - complement fixation,
precipitation, neutralisation of toxin and
viruses
• General purpose antibody
• Protective against infections agents
• Passively administered - suppress
homologous antibody
• IgG late antibody, initial immune response
IgM
• Subclasses - lgG1, lgG2, lgG3, lgG4
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IgA
• Seondmost abundant class
• 10-13% of serum immunoglobulin
• Half-life of 6-8 days
• Major immunoglobulin – colostrum,
saliva, tears
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IgA
• Two forms - Serum lgA, Secretory lgA
• Serum lgA - monomer
• IgA on mucosa, in secretions - dimer
• Two monomeric units - together at the
carboxy terminal by glycopeptide - J chain
(joining)
• Dimeric secretory lgA - synthesised by
plasma cells
• J chain also synthesised by plasma cells
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IgA
Secretory IgA molecule
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SIgA
• Secretory lgA - contains glycine-rich
polypeptide, called the secretory component
or secretory piece
• Produced by mucosal/glandular epithelial
cells
• Dimeric lgA - binds receptor on epithelial
cell, endocytosed transported to luminal
surface
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SIgA
• During the process - part of receptor
remains attached to lgA dimer
• Secretory component
• Secretory piece - protects lgA from
denaturation - by bacterial proteases in the
intestinal mucosa
• SlgA much larger than serum lgA
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SIgA
• Selectively concentrated in secretions and
on mucus surfaces
• Role in local immunity against respiratory
and intestinal pathogens
• Resistant to digestive enzymes
• Inhibit adherence of organism to mucosa
activates - alternate complement pathway
• Promotes phagocytosis and intracellular
killing of microorganisms
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IgM
• 5-8% of serum immunoglobulin
• Half-life of 5 days
• Heavy molecule - millionaire molecule
• lgM – polymer - five peptide subunits
• Polymerisation of subunit depends on J
chain
• Most lgM - intravascular
• Earliest immunoglobulin to be synthesised
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IgM
IgM molecule
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IgM
• Not transported across placenta
• lgM in fetus/newborn - indicates
intrauterine infections
• Short lived, disappear earlier than lgG
• Presence in serum - recent infection
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IgM
• Unique structural features - suited to
biological role
• Protect - microorganism and other large
antigens
• Function lgM lgG
• Immune hemolysis single molecule 1000
molecule
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IgM
• Opsonisation 500-1000 times more
effective than lgG
• Bactericidal action 100 times more
• Bacterial agglutination 20 times more
• Neutralisation of toxin, virus-less active
than lgG
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IgD
• Resembles lgG structurally
• Concentration-3 mg/100 ml serum
intravascular
• Surface of unstimulated B lymphocyte
• Recognition receptor for antigen
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IgE
• 1966 Ishizaka - atopic reagin molecule
• Structurally resemble lgG
• Affinity for surface of tissue cells (mast
cells)
• Mediates- Prausnitz-Kustner reaction
• Does not cross placenta
• Does not fix complement
• Elevated levels – asthma, hay fever, eczema
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IgE
• Produced in lining of respiratory and
intestinal tract
• Responsible for anaphylactic type of
hypersensitivity
• Protection by mast cell degranulation and
release of inflammatory mediators
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ABNORMAL IMMUNOGLOBULIN
• Structurally similar proteins - pathological
processes
• Multiple myeloma – 1847, Bence-Jonesmultiple myeloma light chain
immunoglobulin - kappa or lambda
• In single patient - kappa/lambda, never
both
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ABNORMAL IMMUNOGLOBULIN
• Myeloma - plasma cell dyscrasia
• Unchecked proliferation of one clone of
plasma cells - monoclonal
• Multiple myeloma may affect plasma cell
synthesis of lgG, lgA, lgD, lgE
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ABNORMAL IMMUNOGLOBULIN
• Waldenstrom’s microgloblinemia lgM-producing plasma cells
• Heavy chain disease - lymphoid
neoplasia over production of Fc part of
heavy chain
• Cryoglobulinemia – gel or precipitate
forms on cooling serum
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