IMMUNOLOGY

Detailed explanation-1: -CD28 is a primary costimulation molecule for T cell activation. However, during the course of activation some T cells lose this molecule and assume a CD28-independent existence. These CD28− T cells are generally antigen-experienced and highly differentiated. CD28− T cells are functionally heterogeneous.

Detailed explanation-2: -Thus, the presence of CD28 on the T cells allowed these cells to absorb both Ld and B7-1, presumably in the form of membrane vesicles containing these two molecules. Significantly, the uptake of Ld by CD28+ T cells only occurred when Dros. APCs coexpressed both Ld and B7 (B7-1).