A complex network of specialized cells, tissues, and organs that recognize and defend the body from foreign substances, primarily disease-causing microorganisms such as bacteria, viruses, parasites, and fungi. Organs and tissues of the immune system, called lymphoid organs, include the bone marrow, spleen, thymus, and tonsils. Immune system cells include white blood cells, such as B lymphocytes (B cells), T lymphocytes (T cells), dendritic cells, and macrophages. HIV infection gradually destroys the immune system.