Final answer:
The genetic defect that results in a lack of CD19+ cells and improper B-cell development, leading to an absence of antibody production, is a mutation in the gene for Bruton tyrosine kinase (BTK). This mutation causes X-linked agammaglobulinemia (XLA), where B-cell maturation stops prematurely, resulting in recurrent infections from specific extracellular bacteria.
Step-by-step explanation:
X-Linked Agammaglobulinemia and BTK Mutation
The genetic defect that underlies the condition characterized by a lack of CD19+ cells and improper B-cell development, resulting in an absence of antibody production, is a mutation in the gene for Bruton tyrosine kinase (BTK). This mutation is responsible for a condition known as X-linked agammaglobulinemia (XLA). BTK has a crucial role in B-cell maturation; without it, B-cell development halts at the pre-B-cell stage, preventing the production of immunoglobulin. This leads to patients suffering from recurrent pyogenic infections by extracellular pathogens such as Haemophilus influenzae, Streptococcus pneumoniae, S. pyogenes, and S. aureus.
A mutation in the BTK gene on the X chromosome can cause a considerable deficit in adaptive immunity, severely impacting the immune system's ability to fight infections. While cell-mediated immunity remains intact, the lack of antibody production in these patients leaves them susceptible to certain infections. Understanding the genetic basis of this condition is crucial for diagnosis and management of patients with XLA.