The lymphatic system returns excess tissue fluid to the bloodstream, absorbs fats at the intestinal villi and transports them to the bloodstream, and helps defend the body against disease. The lymphatic system consists of lymphatic vessels, which are structurally similar to veins, and lymphatic organs. The lymphatic organs include the lymph nodes, which house lymphocytes and macrophages, and clean the lymph; the thymus, which houses maturing T lymphocytes; the spleen, which houses lymphocytes and macrophages, and cleans the blood; and the red bone marrow, which produces white blood cells. Immunity is the ability of the body to defend itself against infectious organisms. Specific defenses work against microbes with specific identity markers called antigens. Nonspecific defenses include barriers to entry, the inflammatory reaction, and various protective proteins. The inflammatory reaction occurs as a result of injury and increases capillary diameter and permeability, and marshals phagocytic white blood cells to the site of injury. Protective proteins include the complement system, which destroys bacterial cells by creating holes in their cell walls and plasma membranes; and interferons, which help prepare cells for viral invasion. Specific defense requires the actions of B lymphocytes and T lymphocytes. B lymphocytes provide antibody-mediated immunity by producing antibodies against foreign antigens and T lymphocytes provide cell-mediated immunity by directly attacking cells which bear antigens recognized as foreign to the body. Antibodies produced by B cells bind to specific antigens on the surfaces of microbes, both incapacitating the microbes and marking them for destruction. Immunotherapy deals with manipulating the immune system in order to help individuals avoid or recover from disease. Active immunity develops after individuals have recovered from a disease or have been immunized by vaccination. Active immunity is the product of B lymphocytes called memory cells, which retain the ability to produce antibodies against specific antigens that have been encountered in the past. Passive immunity occurs when an individual is given antibodies to combat a disease. Passive immunity is short-lived because antibodies are not produced by the individual. Allergies are the result of an overactive immune system, and tissue rejection results from T lymphocytes destroying transplanted tissue. Autoimmune diseases involve immune systems that attack body tissues, and immune deficiencies involve immune systems that are incapable of defending against disease. AIDS is an example of an acquired immune deficiency.