Where is the molecular defect in Leukocyte Adhesion Deficiency (LAD) I?

Leukocyte Adhesion Deficiency (LAD) I is primarily associated with a defect in the expression of CD18, which is a component of the beta-2 integrin family. This family includes various integrins that are crucial for leukocyte adhesion to endothelial cells, allowing them to migrate from the bloodstream to sites of infection or inflammation. In individuals with LAD I, mutations in the CD18 gene lead to an inability of leukocytes to properly adhere and transmigrate, resulting in recurrent bacterial infections and impaired inflammatory response.

The importance of CD18 in the context of the immune response highlights how critical integrin molecules are for normal leukocyte function. A deficiency in these molecules disrupts the immune system's ability to respond effectively to pathogens. Other options, like CD15s, CD4, and CD19, are involved in different aspects of immune function but are not implicated in the specific adhesion defects observed in LAD I. CD4 is a marker for helper T cells, CD19 is a B cell marker, and CD15s is associated with myeloid cells. Therefore, their involvement does not contribute to the adhesion issues central to the pathology of LAD I.