Immunodeficiency diseases – AIDS

Immunodeficiency results from the failure of one or more components of the immune system. Primary immunodeficiencies trace back to genetic developmental defects, while secondary immunodeficiencies arise later in life from causes such as radiation, cytolytic or immunosuppressive drugs, or infection. AIDS (Acquired Immuno Deficiency Syndrome) is a secondary/acquired immunodeficiency — not congenital — caused by the Human Immunodeficiency Virus (HIV), a member of the genus Lentivirus. Under the electron microscope HIV appears as a spherical particle 100–120 nm across, with a dense core enclosed in a lipoprotein envelope studded with glycoprotein spikes (gp41 and gp120). Inside the core sit two single strands of RNA, molecules of the enzyme reverse transcriptase, and the enzymes protease and ribonuclease; the core itself is wrapped in a protein capsid, which is in turn covered by a layer of matrix protein.

HIV spreads through unsafe sexual contact, contaminated needles, organ transplants, blood transfusion, and vertical transmission from an infected mother to her child — it is not spread by insects or casual contact, and it survives only about 1.5 days inside a cell but roughly 6 hours outside one. HIV specifically infects helper T cells: after entering the body, it first infects macrophages, where its RNA genome is reverse-transcribed into viral DNA that integrates into the host genome and directs the macrophage to keep churning out new viral particles (effectively turning macrophages into a 'HIV factory'). The virus simultaneously infects helper T lymphocytes, replicates inside them, and releases progeny virus that goes on to infect still more helper T cells — a repeating cycle that progressively depletes the helper-T-cell population. Because infected helper T cells can no longer stimulate B cells to produce antibodies, the person loses natural defence against infection generally, suffers recurring fever, diarrhoea and weight loss, and becomes progressively immune deficient. …