Biology of Disease

and with a Mr of 160 000. However, a proportion of plasma IgA also exists as
a dimer, in which IgA molecules are joined together by a protein called the
joining chain(s) (Figure 4.11 (B)). Secretory IgA exists in this dimeric form
but is protected from enzymic attack by an additional protein known as the
secretory component.

Immunoglobulin E

Immunoglobulin E (IgE) is found in plasma at a concentration of about 5 x 10–5
mg cm–3. Most IgE is found bound to the surface of blood basophils and tissue
mast cells, which possess a high affinity receptor for the heavy chain of this
immunoglobulin. Binding to these cells prolongs the half-life of IgE from two

SPECIFIC IMMUNE RESPONSES

CZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY -(

Figure 4.10 The primary and secondary antibody

responses to an immunogen.

Figure 4.11 Schematics showing the structures of (A) IgM and (B) IgA molecules. J chain is the
joining chain, in each case.

IgM

IgM

IgG

IgG

Secondary response

Primary response

Time / days

0 7 14 21

[Antibody]

S S SS

SS

S

S

S

S

S S

S S

S

S

S

S

S

S

SS

SS

SS

SS

SS SS

S

S

S

S

S

S

J chain

A)

S S SS

SS

SSSS

SS

J chain

B)