Primary Immune Deficiency Syndromes

PRIMARY IMMUNE DEFICIENCY SYNDROMES

X-linked agammaglobulinemia of Bruton is an immunodeficiency characterized bya developmental failure to produce mature B cells and plasma cells, resulting in agammablobulinemia. The condition occurs because of loss of function mutations of B-cell Bruton tyrosine kinase (BTK). Clinically, the disease affects male infants who have recurrent infections beginning at 6 months of life due to the loss of passive maternal immunity. Common infections include pharyngitis, otitis media, bronchi-tis, and pneumonia; common infecting organisms include H. influenza, S. pneumo-coccus, and S. aureus.

Common variable immunodeficiency is a group of disorders characterized by defectsin B-cell maturation that can lead to defective IgA or IgG production. Clinically, both sexes are affected with onset in childhood of recurrent bacterial infections and with increased susceptibility to Giardia lamblia. Complications include increased frequency of developing autoimmune disease, non-Hodgkin lymphoma, and gastric cancer.

DiGeorge syndrome is an embryologic failure to develop the 3rd and 4th pharyngealpouches, resulting in the absence of the parathyroid glands and thymus. Clinical findings can include neonatal hypocalcemia and tetany, T-cell deficiency, and recur-rent infections with viral and fungal organisms.

Severe combined immunodeficiency (SCID) is a combined deficiency of cell-mediatedand humoral immunity that is often caused by a progenitor-cell defect. The modes of inheritance are variable and can include X-linked (mutation of the common [gamma] chain of the interleukin receptors IL-2, IL -4, IL-7, IL-9, IL-15, and IL-21) and autosomal recessive (deficiency of adenosine deaminase). Clinical features include recurrent infections with bacteria, fungi, viruses, and protozoa; susceptibil-ity to Candida, cytomegalovirus (CMV), and Pneumocystis jirovecii infections, and adverse reactions to live virus immunizations. SCID is treated with hematopoietic stem cell transplantation since the prognosis without treatment is death of most infants within a year.

Wiskott-Aldrich syndrome is an X-linked recessive disease with mutation in the genefor Wiskott-Aldrich syndrome protein (WASP). The disease has a clinical triad of recurrent infections, severe thrombocytopenia, and eczema (chronic spongiform dermatitis). Treatment is hematopoietic stem cell transplantation. Complications include increased risk of non-Hodgkin lymphoma and death due to infection or hemorrhage.

Complement system disorders can involve a variety of factors, with deficiencies ofdifferent factors producing different clinical patterns.

In both the classical and alternate pathways, C3 deficiency causes both recurrent bacterial infections and immune complex disease, while C5, C6, C7, and C8 deficien-cies cause recurrent meningococcal and gonococcal infections.

·            In the classical pathway only, C1q, C1r, C1s, C2, and C4 deficiencies cause marked increases in immune complex diseases, including infections with pyogenic bacteria.

·            In the alternate pathway, Factor B and properdin deficiencies cause increased neisserial infections. Deficiencies in complement regulatory proteins can cause C1-INH deficiency (hereditary angioedema), which is characterized clinically by edema at mucosal surfaces with low C2 and C4 levels.

MHC class II deficiency can be caused by defects in positive selection of thymocytes.Few CD4+ lymphocytes develop and as a result, patients suffer from severe immuno-deficiency. Mutations in genes (i.e., CIITA) that encode proteins that regulate MHC class II gene expression are the cause. CD8+ T cells are unaffected.

Hyper IgM syndrome is characterized by normal B and T lymphocyte numbers andnormal to elevated IgM levels but significantly decreased IgA, IgG and IgE levels. Mutations in the gene for CD40 ligand result in the most common form of X-linked hyper IgM syndrome.

Selective IgA deficiency has unknown genetic etiology. Many affected individu-als appear healthy while others have significant illness. Sinopulmonary infections, diarrhea and adverse reactions to transfusions can occur. Levels of IgA are unde-tectable whereas levels of other isotypes are normal. There is an association with autoimmune disease.