Involved in the costimulatory signal essential for T-lymphocyte activation. T-cell proliferation and cytokine production is induced by the binding of CD28, binding to CTLA-4 has opposite effects and inhibits T-cell activation. (Microbial infection) Acts as a receptor for adenovirus subgroup B. Homodimer; CD80 dimers on the antigen presenting cells (APCs) bridge CTLA4/CD152 dimers on T-cells in a periodic zipper-like arrangement. (Microbial infection) Interacts with adenovirus subgroup B fiber proteins. Expressed on activated B-cells, macrophages and dendritic cells. Soluble isoform. Expressed in unstimulated B-cells and monocytes, but not T-cells. Soluble isoform. Expressed in T-cells activated by ConA, non-activated monocytes and monocytes activated with IFN-c.