In the present paper the authors review the biological properties of the HIV infection. The preferential target of HIV infection is the lymphocyte that expresses the CD4 phenotype. Infection of CD4 subsets leads to a remarkable cytopathic effect that can be blocked by antibodies anti CD4 epitope. Actually the HIV envelope proteins recognize an epitope of the CD4 surface molecules; as consequence the antibodies anti idiotypic directed to CD4 indirectly present the binding of envelope proteins to CD4. The relationship between this retrovirus and the immunological pathogenesis of the disease are also discussed. HIV infection is facilitated in groups affected by alterations of the immune system as drug abusers, haemophiliacs and homosexuals. Secondary immunological abnormalities are consistently pronounced in people seropositive for the virus and affected by a progressive generalized lymphoadenopathy (PGL). The degeneration of primary lymphatic organs causes a reduction of the T-cell response against any antigenic challenge. After the PGL phase the disease shifts on AIDS, that is an irreversible stadium accompanied by the deterioration of the clinical status and by an increase of immunosuppression that favours the arise of tumours. In addiction HIV infection may cause severe central nervous system (CNS) dysfunctions, the HIV associated neurological syndromes include myelopathy, meningitis, encephalitis and peripheral neuropathy. HIV has also been isolated from brain tissue and from cerebral spinal fluid.