SN5 SHORT NOTE 5M

Describe the structure, classes and functions of the immunoglobulins.

Biochemistry · 1st Professional — Pre-Clinical

Immunoglobulins (antibodies) = glycoproteins produced by plasma cells that recognise and bind specific antigens.

Y-shaped; four polypeptide chains — two identical heavy and two identical light chains
Held by interchain disulphide bonds
Light chains — two types, kappa (κ) and lambda (λ); a given molecule has one or the other, never both
Heavy chains determine the class — γ, α, μ, δ, ε for IgG, IgA, IgM, IgD, IgE
Variable region — antigen binding; constant region — effector functions
Hinge region gives flexibility; rich in proline

Enzymatic fragments

Enzyme	Fragments produced	Properties
Papain	2 Fab + 1 Fc	Each Fab is monovalent — binds but cannot precipitate
Pepsin	1 F(ab′)2 + fragments	Bivalent — still precipitates antigen; Fc destroyed

Class	Structure	Proportion	Chief features
IgG	Monomer	75–80%	The only one crossing the placenta; fixes complement; secondary response; longest half-life (23 days)
IgA	Monomer in serum; dimer with secretory piece in secretions	10–15%	Chief antibody of mucosal secretions — milk, saliva, tears, gut
IgM	Pentamer	5–10%	Largest; first in the primary response; best complement fixer; natural blood group antibodies
IgD	Monomer	< 1%	B cell surface receptor
IgE	Monomer	Least (0.004%)	Allergy and parasitic infection; binds mast cells and basophils

CLINICAL PEARL

Mnemonics — IgG Goes across the placenta; IgA in secretions (sAliva); IgM is Massive and first; IgE for Exposure and allergy.

Neutralisation of toxins and viruses
Opsonisation — coating organisms for phagocytosis
Complement activation — classical pathway; IgM > IgG
Agglutination and precipitation
Antibody-dependent cell-mediated cytotoxicity
Passive immunity to the newborn — IgG across the placenta, IgA in breast milk

Multiple myeloma — a monoclonal plasma cell tumour producing a sharp M band on electrophoresis; free light chains appear in urine as Bence Jones protein
Raised IgM alone in the newborn indicates intrauterine infection, since IgM cannot cross the placenta and must be the infant's own
Rh incompatibility — maternal IgG crosses the placenta and causes haemolytic disease; prevented by anti-D immunoglobulin
Monoclonal antibodies are now major drugs — rituximab, trastuzumab, infliximab, adalimumab
Selective IgA deficiency is the commonest primary immunodeficiency; recurrent mucosal infection and a risk of anaphylaxis on transfusion

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