The immune system is the body's defense against pathogens. Cells and messenger substances that are specifically designed for this purpose fight the invaders and save certain blueprints for a later date in order to be able to react more quickly to recurring pathogens. The immune system includes organs such as bone marrow, thymus, spleen, tonsils, lymph nodes and special white blood cells. These cells of the immune system are called leukocytes and are divided into different cell types such as granulocytes, macrophages and natural killer cells (NK cells).
