Immunology studies the human body’s defense mechanisms against foreign subjects such as viruses and bacteria. The primary immune system organs are the thymus and bone marrow, both of which produce immune cells. Other organs or tissues such as spleen, lymph nodes and vessels, tonsils, adenoids and skin also participate in our body’s immune defense. The immune system is divided into an innate immune system, which provides immediate but non-specific response to pathogens, and an adaptive immune system, which is mediated by immune cells such as T and B lymphocytes and only responds to foreign subjects that they previously encountered.