Once in the bloodstream, HIV attacks certain types of cell, mainly T lymphocytes. The envelope proteins of the virus bind to two receptors (CD4 and a closely positioned co-receptor) on the surface of these cells. The interaction of the envelope proteins with the two receptors triggers fusion of the virus membrane to the host cell membrane, allowing the virus to enter the host cell. The virus core is released into the cytoplasm, leaving the viral membrane and envelope proteins behind on the outer surface of the cell. Inside the cell the core protein is dissolved by host cell enzymes, releasing viral RNA and viral enzymes (reverse transcriptase, integrase and RNAse-H) into the cytoplasm.