In 1991, three groups reported discovering CD154. Seth Lederman, Michael Yellin, and Leonard Chess at Columbia University generated a murine monoclonal antibody, 5c8, that inhibited contact-dependent T cell helper function in human cells and which characterized a 32 kDa surface protein transiently expressed on activated CD4+ T cells. Richard Armitage at Immunex cloned a cDNA encoding CD154 by screening an expression library with CD40-Ig. Randolph Noelle at Dartmouth Medical School generated an antibody that bound a 39 kDa protein on murine T cells and inhibited helper function. Noelle contested Lederman's patent, but the challenge (called an interference) was rejected on all counts 
CD40 ligand is primarily expressed on activated CD4+ T lymphocytes but is also found in a soluble form. While CD40L was originally described on T lymphocytes, its expression has since been found on a wide variety of cells, including platelets, mast cells, macrophages, basophils, NK cells, B lymphocytes, as well as non-haematopoietic cells (smooth muscle cells, endothelial cells, and epithelial cells).
In the macrophage, the primary signal for activation is IFN-γ from Th1 type CD4T cells. The secondary signal is CD40L on the T cell, which binds CD40 on the macrophage cell surface. As a result, the macrophage expresses more CD40 and TNF receptors on its surface, which helps increase the level of activation. The activated macrophage can then destroy phagocytosed bacteria and produce more cytokines.
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Xu Y, Song G (2005). "The role of CD40-CD154 interaction in cell immunoregulation". Journal of Biomedical Science. 11 (4): 426–38. doi:10.1159/000077892. PMID15153777.
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