**1. Introduction**

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HIV is known to be associated with a wide range of immunological and haematological changes.The immunological changes include depletion in CD4+ T cell, cytokine dysregulation and immune dysfunction. The dominant immunologic feature of HIV infection is progressive depletion of the helper T cell (CD4+ T cell), which reverses the normal CD4:CD8 ratio and subsequently lead to immunodeficiency. CD4+ T cells interact with antigen presenting cells (APCs), B cells, cytotoxic T cells (CD8+ T cells) as well as natural killer cells. Thus, infection and depletion of CD4+ T cell population could induce profound immunodeficiency in such patients.

The haematological changes occur mainly due to several factors such as marrow defects and immune cytopenias. It is caused by HIV infection, either directly to the bone marrow, oppor‐ tunistic infections, development of lymphoma as a secondary neoplasm and side effects of the drugs used for the treatment or drugs used for the complicating infection or lymphoma. In order to further understand the immunological and haematological changes that occur in HIV infection, the chapter begins with the review of immune system as well as normal haemato‐ poiesis. It further highlights the importance of changes associated with clinical symptoms in patients with HIV.
