Chemokine Receptors and Leukocyte Trafficking
A chemokine gradient does not move a leukocyte anywhere. It only tells one that has already stopped which way to go. Trafficking is a four-step cascade and the steps are not interchangeable: tether and roll on selectins, then inside-out integrin activation triggered by a chemokine receptor, then firm arrest, and only then diapedesis. Block arrest and no gradient in the world will get the cell into tissue — which is exactly how natalizumab and vedolizumab work, and why they are so tissue-selective. This map is the positioning signal for every other pathway in this library: the cells in all thirty-eight of the other maps had to get where they are somehow. Click any protein for the matching Assay Genie ELISA kit or In Vivo antibody.
The order of the steps is the whole mechanism. Selectins catch a fast-flowing cell and convert it to a slow roll — low-affinity, high-off-rate bonds that survive shear precisely because they break and reform. A rolling cell then encounters chemokine displayed on the endothelial surface, and its receptor triggers RAP1 and talin-1 to switch the integrin from a bent, low-affinity conformation to an extended, high-affinity one. That is inside-out signalling, and it happens in under a second. Only then does firm arrest occur. Miss this ordering and the experiment makes no sense: a chemokine-receptor antagonist and a selectin blocker both reduce recruitment, but they fail at different steps and give different tissue selectivity.
Redundancy is the reason single-chemokine blockade almost never works. CXCL9 and CXCL10 both signal through CXCR3; CXCL8 uses both CXCR1 and CXCR2; CCL19 and CCL21 both use CCR7. Remove one ligand and the others cover for it. Blocking at the receptor is more effective than blocking at the ligand, which is the same lesson the VEGF map teaches. ACKR1 (the Duffy antigen) is on the map as the exception that proves the design: it binds many chemokines and signals through none of them, acting as a scavenger and a transcytosis platform. Measuring chemokine in a tissue where ACKR1 is abundant tells you about sequestration, not signalling.
Arrest is where the drugs are, and it is where the tissue selectivity lives. The integrin determines the destination because the ligand is expressed regionally. CD18 paired with CD11a (LFA-1) against ICAM-1 is general-purpose. CD49d (VLA-4 α chain) against VCAM-1 gets cells into brain and inflamed tissue — natalizumab blocks exactly this, which is why it works in multiple sclerosis and why it carries a PML risk: it also blocks immune surveillance of the CNS. Pair the same α4 chain with integrin β7 instead and you get α4β7 (LPAM-1), which binds MAdCAM-1 on gut endothelium only — vedolizumab's target, and the reason it is effective in inflammatory bowel disease with none of the CNS liability. One shared α chain, two partners, two completely different safety profiles.
Then the cell has to stay. Egress from tissue requires S1PR1, and CD69 physically sequesters S1PR1 to prevent it — which is the mechanism fingolimod exploits from the other direction, and the reason CD69 is the canonical residency marker rather than merely an activation marker. CCR9 and CD103 complete the gut-tropic address. The In Vivo tie-in: 13 of 37 nodes carry a functional-grade antibody, and they sit exactly where the interventions are: CD11a and CD18 for LFA-1, CD49d, integrin β7 and LPAM-1 for the α4 pair, ICAM-1 in both mouse and human, L-selectin in both species, CD44, CXCR4, CXCR3 and CXCL9. That is enough to block any single step of the cascade in vivo and ask which one your model actually depends on — which is the experiment this map exists to make obvious. For research use only; not for use in diagnostic or therapeutic procedures.
Every protein node links to a product — ELISA kit, In Vivo antibody or research antibody.