Complex sentence: CD154, also called CD40 ligand or CD40L, is expressed on T cells surfaces and combines with CD40 on antigen presenting cells to induce antibody isotype switching in B cells.
Split into simpler sentences: CD154, also called CD40 ligand or CD40L, is expressed on T cells surfaces. It combines with CD40 on antigen presenting cells to induce antibody isotype switching in B cells.