CD4 lymphocytes are T helper lymphocytes that present in their plasma membrane receptor proteins called CD4. CD4 lymphocytes are the cells that HIV infects and within which the virus replicates. HIV has proteins in its capsule that bind to the CD4 receptors of lymphocytes. Through that bond the virus fuses with the cell membrane and its content (RNA, reverse transcriptase, protease, etc.) penetrates into the cytoplasm and the viral replication process begins.
HIV RNA is then converted into DNA by the reverse transcriptase. The new DNA is inserted into the genetic material of the lymphocyte with the aid of enzymes called integrases. By transcription and translation this DNA commands the synthesis of proteins necessary for the assemblage of new viruses. Long polypeptides are thus produced and then fragmented into proteins and viral enzymes by the enzyme protease. So new HIV viruses are assembled and break the cell membrane to gain the circulation.