Which cell surface molecules are absent in X-linked agammaglobulinemia?

In X-linked agammaglobulinemia (XLA), there is a significant defect in B-cell development due to mutations in the BTK gene. This impairment leads to a marked reduction in B cells, which directly affects the expression of certain cell surface molecules that are typically present on B lymphocytes.

CD19 and CD20 are crucial markers for B cells. CD19 is involved in B-cell signaling and is expressed on all stages of B-cell development, while CD20 plays a role in B-cell activation and differentiation. In individuals with XLA, the absence of functional B cells means that these markers will not be present on the surface of the affected cells.

The presence of other options like CD4, CD8, CD45, CD32, CD3, and CD28 relates mainly to other types of immune cells, such as T cells or are associated with different functions and cell lineages. Therefore, the main reason why the correct answer identifies CD19 and CD20 is that their expression is directly tied to the functionality and presence of B cells, which are critically lacking in X-linked agammaglobulinemia.