ABSTRACT
This chapter describes the X-linked immunodeficiencies, outlines the nature of the diseases, the defective genes and present state of understanding of the molecular pathogenesis of the clinical and immunological phenotypes. The phenotype of XLA is characterized in its classical form by the almost complete absence of immunoglobulin of all isotypes and the profound reduction of B-lymphocytes in the peripheral circulation. The identification of the genetic abnormality in XLA has allowed unambiguous assignment of molecular defects to individuals with abnormalities in antibody production. IL-2 binding to high affinity receptor complexes was severely reduced and T-cells from affected individuals showed impaired in vitro stimulation and a restricted T-cell receptor repertoire. Signalling lymphocyte activating molecule (SLAM) is a B- and T-cell surface marker which forms a receptor-ligand pair, triggering of which co-activates B- and T-lymphocyte responses. Properdin deficiency is characterized by absence of extracellular properdin, a positive regulator of the alternative pathway of complement activation.
