Review Question - QID 100478

The lack of immunoglobulins and the recurrent bacterial infections indicate a deficiency in B cell-mediated immunity. CD19 is an important cell-surface receptor present on B-cells.

The patient is likely suffering from X-linked agammaglobulinemia. In this disease, the Bruton tyrosin kinase (Btk, on X-chromosome) gene is mutated, resulting in impairment of the pre-B cell receptor and an arrest in pre-B cell maturation. The result is the absence of mature B-cells and agammaglobulinemia. Symptoms such as those described in the question typically appear around 6 months when most/all maternal immunoglobulins have been degraded. An absence of CD19+ cells confirms diagnosis; CD19 is a component of the B-cell co-receptor (CD19, CD21, and CD81). When the co-receptor and B-cell receptor (composed of an Ig and CD79) cluster during antigen binding, CD19 is phosphorylated intracellularly and initiates a cascade that helps lead to B-cell activation (along with BCR signals).

Mohamed et al. give an overview of Btk and stress its importance in B cell signaling. Btk is a cytoplasmic tyrosine kinase which is part of a signaling cascade that ultimately results in calcium mobilization, cytoskeletal rearrangement, and transcriptional regulation of NF-Kappa-B and NFAT.

Cooper et al. discuss primary immunodeficiency and note that although Brunton's agammaglobulinemia usually presents by the end of the first year of life with recurrent bacterial infections, some patients can live until the age of 3-5 years without any symptoms.

Illustration A diagrams the mechanism of action of a B cell.

Incorrect Answers:
Answer 1: CD4 is found on T-helper lymphocytes.
Answer 2: CD8 is found on cytotoxic T cells.
Answer 3: CD3 is found on all T cells.
Answer 5: NKG2D is found on NK cells and CD8 T cells.