They prevent macromolecules from navigating the intercellular cleft and limit the lateral diffusion of intrinsic membrane proteins and lipids between the apical and basolateral cell surface domains. In the intercellular clefts of capillaries, tight junctions are the first structural barriers a neutrophil encounters as it penetrates the interendothelial cleft, or the gap linking the blood vessel lumen with the subendothelial space2. In capillary endothelium, plasma communicates with the interstitial fluid through the intercellular cleft.