The HIV virus grows in lymphocytes, monocyte-macrophages and possibly a few other cells of the human body. This is an RNA virus that attaches at the CD4 receptor, enters the cell, and makes DNA by using a reverse transcriptase enzyme. The virus is about 100 nm in diameter and expresses several surface and core antigens. Growth of the virus in the helper (T4) lymphocytes results in destruction of these cells. This, in turn, results in loss of cellular immune functions that causes susceptibility to infections and malignancies. Monocyte-macrophage cells are also infected but these cells are not lysed. Antigen to HIV appears in blood a few weeks after infection but disappears as antibody begins to be present. Antibody can be detected by a variety of tests but ELISA and Western blot assays are used most frequently. New latex and other methods are being licensed for use under certain circumstances.