Why do you think two signals are required for B- and T-cell activation but only one signal is required for activation of an APC?

A cell that displays antigen bound by major histocompatibility complex (MHC) proteins on its surface is known as an antigen-presenting cell (APC) or accessory cell. The T cell receptors on T cells may be able to identify these complexes. T-cells are presented with antigens by APCs. Antigens can be presented in various ways by nearly all cell types. They can be found in many different types of tissue. 

Antigen-activated B cells can also deliver the requisite costimulatory molecules to activate the T cell's CD28 receptor, in addition to their ability to choose antigen-correct T cells. To activate 56 T cells, you'll need two signals. The first signal is delivered by their antigen receptor, whereas the second signal is provided by APCs such as monocytes, macrophages, dendritic cells, or B cells, and is commonly provided by CD28. T cells do not mount an immune response to self-antigens because of the two-signaling requirement. T cells have no way of knowing which antigens are harmful and which are not.