Immunoglobulin G4 and related diseases.

i mmunoglobulins, produced by plasma cells, are the effector arm of the humoral immune system [1]. Each immunoglobulin consists of two identical heavy chains (α, β, γ, δ or ε – all encoded by chromosome 14) and two identical light chains (either κ or λ, encoded by chromosome 2 and 22, respectively) [2]. The immunoglobulins have constant (C) regions (similar within the same isotype) and variable (V) regions that define the binding specificity of the immunoglobulins [Figure 1]. There are nine immunoglobulin isotypes: IgG11, IgG2, IgG3, IgG4, IgM, IgA1, IgA2, IgD and IgE. All immunoglobulin isotypes are present in the serum of normal individuals [3]. There is a 90% homology between the four IgG isotypes, whereas between IgG and IgM isotypes the homology is about 30% [4]. There