1) Antibodies possess both an antigen binding capacity and a biological activity. Digestion with enzymes like papain and pepsin can yield fragments that retain one or the other function. 2) Papain digestion yields two Fab fragments that retain antigen binding and an Fc fragment that retains biological activity. Pepsin digestion yields an F(ab')2 fragment with two antigen binding sites and degrades the Fc fragment. 3) The major classes of antibodies in mammals are IgG, IgM, IgA, IgD, and IgE, which differ in their heavy chain constant regions. IgG is the most abundant in serum and provides a major defense against bacteria, viruses, and parasites through mechanisms like

1. Immunology
Structure Function
Relationship of
Antibodies
By
Ahmed Riyadh Abdul Rahman
Al-Noor University College
1

2. Structure Function Relationship of Antibodies
• Antibodies possess both an antigen binding capacity and a biological
activity.
• This bifunctional nature of antibodies was initially demonstrated by
proteolytic cleavage (digestive) of antibody.
2

3. 1-Digestion of antibodies with papain
 yields three fragments, two copies of a single antigen binding region, F(ab),
and one readily crystallizable fragment (Fc) which cannot bind antigen.
 Papain cleaves immunoglobulin G molecules behind the hinge region
(disulfide bond) which results in the generation of three ~50kDa fragments;
two Fab fragments (~100kDa) and an Fc fragment (~50kDa), (total
molecular weight of IgG molecule ~150 kDa).

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2-Digestion of antibodies with pepsin
 Digestion of immunoglobulins by pepsin occurs in front of hinge region
(disulfide bond i.e. below the hinge region) of the heavy chains, The resulting
F(ab')2 fragment is comprised of two Fab units joined by disulfide of their
heavy chains at the hinge region.
 F(ab')2 possessing two antigen binding sites, that can bind two antigen
molecules. In pepsin digests, the Fc portion is proteolytically degraded so it
loses its biological activity.

5. Antibody IgG
structure and
cleavage sites
for
fragmentation
by papain &
pepsin

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Immunoglobulin Classes and Subclasses
 Immunglobulin molecules are divided into distinct classes and
subclasses in terms of the differences in amino acid sequence of
constant region of heavy chain, i.e. γ,α ,ε , μ, and δ.

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Immunoglobulin Classes of Mammals
1. IgG – Gamma (γ) heavy chains.
2. IgM – Mu (μ) heavy chains.
3. IgA – Alpha (α) heavy chains.
4. IgD – Delta (δ) heavy chains .
5. IgE – Epsilon (ε) heavy chains.

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1. Immunoglobulin G (IgG)
 Immunoglobulin G, or IgG, is the major immunoglobulin in human
serum, accounting for 70-80% of the total immunoglobulin in serum.
 IgG mostly found in serum and extracellular fluid for example in
pleural fluid, Synovial fluid, Peritoneal fluid,…etc.
 IgG is a monomer consisting of identical pairs of H and L chains linked
by disulfide bond.
 Each IgG has two antigen binding sites (divalent).

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 Sedimentation coefficient of IgG is (7s), and molecular weight about~150-
180 kDa.
 There are four human IgG isotypes (IgG1, IgG2, IgG3, and IgG4); based on
amino acid sequence differences in the H chain constant region and on the
number and location of disulfide bonds.
 IgG is the only immunoglobulin that can cross the placenta in humans and
protect the infant during the first months of life.
 IgG has a half – life of approximately (21) days .

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 IgG is the predominant (the major) antibody in secondary immune responses
and constitutes an important defense against bacteria (Antibacterial), viruses
(antiviral) and parasite (antiparasite).
 IgG plays a major role in (i) opsonization by phagocytes; (ii) antibody-
dependent cell mediated cytotoxicity (ADCC) by natural killer cells; (iii)
complement activation; and (iv) neutralization of viruses and toxins(ѵ)
agglutinating and heamagglutinating activity.
 IgG the most effective Ig in immunity
 IgG affected by the proteolytic enzyme.

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IgG plays a major role in opsonization by phagocytes

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IgG an important defense
against viruses (antiviral activity)

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IgG plays a major role
in antibody-dependent
cell mediated
cytotoxicity (ADCC)
by natural killer cells

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neutralization of viruses and toxins complement activation

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