Q.31 A 4-mоnth-оld Greek bаby girl is аdmitted tо the hospitаl with severe bacterial sepsis. Labs show markedly decreased serum immunoglobulin levels (B-cells are not producing enough antibodies). Flow cytometry shows very few CD4+ T helper (Th) cells. In vitro testing reveals that the baby’s dendritic cells are unable to effectively activate CD4+ Th cells. Absence of which of the following molecules on her dendritic cells most likely accounts for her poor immunologic response to a bacterial infection?