B-Cell Immunodeficiencies

B-Cell Immunodeficiencies

B-cell deficiencies include (a) X-linked hypogammaglobu-linemia, (b) selective immunoglobulin deficiencies, (c) hyper-IgM syndrome, and (d) interleukin-12 receptor deficiency.

◗ X-linked hypogammaglobulinemia

X-linked hypogammaglobulinemia, or infantile agammaglob-ulinemia or X-linked agammaglobulinemia (XLA), is the pro-totype of “pure” B-cell deficiency. In the majority of cases, the disease is transmitted as a sex-linked trait. The defective gene is located on Xq21.2–22, the locus coding for the B-cell pro-genitor kinase or Bruton’s tyrosine kinase (Btk). Btk plays an important role in B-cell differentiation and maturation, and is also part of the group of tyrosine kinases involved in B-cell signaling in adult life. Most cases of infantile agammaglobu-linemia are associated with mutations affecting Btk. X-linked hypogammaglobulinemia shows the follow ing features:

·           It is characterized by extremely low IgG levels and by the absence of other immunoglobulin classes.

·           Individuals with XLA have no peripheral B cells and suf-fer from recurrent bacterial infections, beginning at about 9 months of age. Patients suffer from repeated infections caused by common pyogenic organisms (Streptococcuspneumoniae, Neisseria meningitidis, Haemophilus influenzae, Staphylococcus aureus, etc.) causing pyoderma, purulent con-junctivitis, pharyngitis, otitis media, sinusitis, bronchitis, pneumonia, empyema, purulent arthritis, meningitis, and septicemia. Chronic obstructive lung disease and bronchiec-tasis develop as a consequence of repeated bronchopulmo-nary infections. Infections with Giardia lamblia are diagnosed with increased frequency in these patients and may lead to chronic diarrhea and malabsorption.

Agammaglobulinemic patients are at risk of developing para-lytic polio after vaccination with the attenuated virus; they also are at risk of developing chronic viral meningoencephali-tis, usually caused by an echovirus. Arthritis of the large joints develops in about 30–35% of the cases and is believed to be infectious, caused by Ureaplasma urealyticum. This condition is best treated with replacement therapy using gamma globulin (a plasma fraction containing predominantly IgG, obtained from normal healthy donors) administered intravenously.

◗ Selective immunoglobulin deficiencies

In this condition, only one or more of the immunoglobulins are deficient in serum, while the others remain normal or ele-vated. IgA deficiency is the most common example of selective immunoglobulin deficiencies. IgA deficiency is characterized by nearly absent serum and secretory IgA. The IgA level is less than 5 ng/dL, but the remaining immunoglobulin class levels are normal or elevated. The disorder is either familial or it may be acquired in association with measles or other types of viral infection, or toxoplasmosis.

The etiology of IgA deficiency is unknown, but is believed to be due to arrested B-cell development. The principal defect appears to be in IgA B-cell differentiation. The adult patients with selective IgA deficiency usually express the immature phe-notype, only a few of which can transform into IgA-synthesizing plasma cells. Although IgA cells are produced, these cells fail to secrete IgA.

IgA is the principal immunoglobulin in secretions and is an important part of the defense of mucosal surfaces. Thus, IgA-deficient individuals have an increased incidence of respi-ratory, gastrointestinal, and urogenital infections. They also have an increased incidence of autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthri-tis. There is an increased incidence of the disorder in certain atopic individuals. Some selective IgA-deficiency patients form significant titers of antibody against IgA. They may develop anaphylactic reactions upon receiving IgA-containing blood transfusions.

Selective IgA deficiency is diagnosed by the demonstration of less than 5 mg/dL of IgA in serum. They, however, have normal levels of IgG and IgM antibodies. Some individuals develop antibodies against IgG, IgM, and IgA.

◗ Hyper-IgM syndrome

This condition is characterized by high concentration of serum IgM but very low concentration of serum IgG, IgA, and IgE. They have normal numbers of T cells and B cells. Some of these immunodeficiencies are X-linked and some are inherited as autosomal recessives. Patients with this condition are suscep-tible to recurrent microbial infections and many autoimmune disorders, such as thrombocytopenia, neutropenia, and hemo-lytic anemia.

◗ Interleukin-12 receptor deficiency

Patients with interleukin-12 receptor deficiency are highly susceptible to disseminated mycobacterial infections. Lack of interleukin-12 receptor prevents IL-12 initiating a Th-1 response, which is essential to prevent mycobacterial infections.