Abstract
We used immunogold label-fracture to follow the migration of HLA I class and HLA II class antigens during capping as induced by specific monoclonal antibodies. Capping is achieved through a process of clustering and "consolidation" of clusters into larger patches and, finally, a single cap. All receptors appear to cluster from the very start, with no "stray" molecules joining already formed patches. Characterization of exoplasmic and protoplasmic fracture-faces of capping cells fails to reveal any corresponding accumulation of intramembrane particles and/or subtler rugosities. Our results are consistent with the concepts that view the migration of capping molecules as contemporaneous with the efflux of noncapping integral membrane proteins.