Essentially, the immune system is a network of cells, tissues, and organs that are designed to protect the body from outside invaders. Infections are primarily caused by microbes, which include bacteria, parasites, and fungi. Viruses also cause infections, but they are too primitive to be classified as living organisms. Humans provide a perfect environment for a wide range of bacteria and viruses. As a matter of fact, the immune system should seek out and destroy them, if that doesn't work. Immunity is one of the most complex systems in the human body. Neither an organ nor a cell can be considered a part of it. There are many different types of cells and organs that make up the immune system. There are two main groups of these organs based on their function. Lymphocytes are trained to discriminate between self and non-self-antigens in the primary lymphoid organs by providing appropriate microenvironments. As the primary lymphoid organ in humans, the bone marrow produces B-cells, whereas the thymus produces t-cells.