Immunodeficiency Due To Selective Anti-Polysaccharide Antibody Deficiency

Immunodeficiency due to selective anti-polysaccharide antibody deficiency is characterized by normal immunoglobulin levels (including IgG sub-classes) but impaired polysaccharide responsiveness (IPR).

Epidemiology

Although the prevalence is not really known, around 100 cases have been reported in the literature, indicating that this syndrome is a rare primary immunodeficiency. Approximately 60% of patients are male.

Clinical description

The onset of the disease generally occurs during childhood, between 2 to 7 years of age. Patients suffer from recurrent bacterial infections, mostly of the respiratory tract. The offending bacteria are those with a polysaccharide capsule, such as pneumococci, Hemophilus influenzae, meningococci and group B streptococci. Sepsis and meningitis occur less frequently. Allergic manifestations are observed in half of the patients.

Etiology

This immunodeficiency is likely heterogeneous with multiple causes. Genetic factors may play a role, as indicated by the observation of a higher prevalence in certain ethnic populations and of some familial cases. Several hypotheses have been proposed concerning the cause of the disease, but the most likely is a defect in splenic marginal zone B cells. Indeed, polysaccharide antigens are concentrated and presented to B cells by the dendritic cells within the spleen marginal zone. In favour of this hypothesis is the observation of an impaired polysaccharide antibody response in splenectomized patients.

Diagnostic methods

The diagnosis is established by identifying deficient antibody response to polysaccharide antigens (usually Haemophilus influenzae b vaccine) contrasting with normal immunoglobulin (including the IgG subclasses) levels and unaffected antibody production to protein antigens (tetanus toxoid, diphtheria). As most children under 2 years of age have a physiological defect in response to polysaccharide antigens, the diagnosis cannot be assessed before this age.

Differential diagnosis

The differential diagnosis should exclude other primary immunodeficiencies also characterized by a defective response to polysaccharide antigens, essentially the IgG2-IgG4 deficiency. A defect in antibody production to polysaccharides may be associated with the Wiskott-Aldrich syndrome or Common Variable Immuno Deficiency (CVID). Recently, an adult patient with Btk-deficiency has been reported as only affected by impaired polysaccharide responsiveness.

Management and treatment

Besides their use for controlling infections, antibiotics should also be given as a prophylactic treatment. Immunoglobulin substitution could also be of benefit whenever prophylactic antibiotherapy fails. Vaccination with the conjugate antipneumococcal vaccine is also required.

Prognosis

Under treatment, infections are generally well controlled. However, patients should be carefully followed-up since this condition can evolve into a more severe immunodeficiency (IgG subclass deficiency or CVID).