Human Immunity and Physiology
Immune system is a complex system that is responsible for protecting us against infections and foreign substances. There are three lines of defense: the first is to keep invaders out, the second line of defense consists of non-specific ways to defend against pathogens that have broken through the first line of defense. The third line of defense is mounted against specific pathogens that are causing disease. The immune system is closely tied to the lymphatic system, with B and T lymphocytes being found primarily within lymph nodes. Tonsils and the thymus gland are also considered lymph organs and are involved in immunity. We often don't realize how effective the immune system is until it fails or malfunctions, such as when the lymphocytes are attacked by HIV in an AIDS patient.