Immunity is defined as resistance to diseases, especially infectious diseases. The set of cells, tissues and molecules that mediate resistance to infection is called the immune system, and the coordinated response of these cells and molecules to infectious microbes comprises the immune response. Host defenses are grouped under innate immunity, which provides immediate protection against invading microorganisms, and acquired immunity, which develops more slowly and provides a more specialized defense against infection. Innate immunity, which is also called non-specific immunity, is always present in healthy individuals and is ready to block the entry of microorganisms and quickly eliminate microorganisms that manage to enter the host's tissue. Acquired immunity, also called specific immunity, requires the proliferation and differentiation of lymphocytes in response to microorganisms before it can provide an effective defense; that is, it adapts to the presence of microbial invaders. Autoimmunity is generally defined as a phenomenon in which antibodies or T cells react with autoantigens, and autoimmune diseases arise as a result of disturbances in the sense of immunity to one's own antigens, manifested by damage to one's own tissue. An excessive immune reaction can cause a harmful inflammatory response, and this is immune hypersensitivity. An allergy or hypersensitivity condition occurs when the immune system reacts to harmless substances in the environment, such as food, grass pollen, house dust or pet dander. The consequence of the lack or absence of elements of the immune system, including lymphocytes, phagocytes and the complement system, is called immunodeficiency. The human immunodeficiency virus (HIV) is a virus that infects cells of the immune system, mainly CD4+ T lymphocytes, and causes the progressive destruction of these cells. Immunocompromised patients may require longer hospitalization and rehabilitation, therefore the nurse plays an important role in health care.