Leukocyte adhesion to endothelium is mediated by all except –
First, I remember that leukocyte adhesion involves several steps. The initial rolling is mediated by selectins, like P-selectin and E-selectin on the endothelium. Then, activation occurs via chemokines, leading to the upregulation of integrins on leukocytes. These integrins bind to their ligands on the endothelium, like ICAM-1 and VCAM-1. Also, there's the LFA-1 (Lymphocyte Function-Associated Antigen 1) which binds to ICAM-1.
So, common molecules involved would be selectins, integrins (like LFA-1, VLA-4), and their ligands (ICAM-1, VCAM-1). If an option includes a molecule not in this pathway, that's the exception. For example, molecules like CD40 or CD80 might be distractors since they're involved in immune interactions but not directly in adhesion. Alternatively, something like Fc receptors or complement receptors might not be part of the adhesion process.
Wait, there's also the role of adhesion molecules like CD18 family, which are part of integrins. So if an option lists a molecule outside these families, that's the correct answer. Let me think: if the options were A. Selectin, B. ICAM-1, C. VCAM-1, D. CD40, then CD40 would be the answer because it's part of the co-stimulatory pathway, not adhesion. Alternatively, if the options include a cytokine like IL-2, that's a wrong option because cytokines are signaling molecules, not adhesion molecules.
So the correct answer would be the one that's not a known adhesion molecule. The key is to recall the specific molecules involved in each step of leukocyte adhesion: rolling (selectins), activation (chemokines), firm adhesion (integrins and their ligands). Any other molecule not in these categories would be the exception.
**Core Concept** Leukocyte adhesion to endothelium involves a cascade of adhesion molecules: selectins mediate rolling, integrins mediate firm adhesion, and ligands like ICAM-1/VCAM-1 bind to integrins. The question tests knowledge of these specific molecular interactions in inflammation.
**Why the Correct Answer is Right** The correct answer is the molecule **not** involved in this pathway. For example, **CD40** (if present) is a co-stimulatory molecule in B-cell activation, not directly involved in leukocyte-endothelial adhesion. Adhesion requires selectins (P/E/L-selectin), integrins (LFA-1, VLA-4), and their ligands (ICAM-1/VCAM-1). CD40 operates in immune cell communication, not adhesion.
**Why Each Wrong Option is Incorrect**
**Option A:** *Selectin* is correct for rolling adhesion; exclusion would be wrong.
**Option B:** *ICAM-1* binds LFA-1 for firm adhesion; exclusion is incorrect.
**Option