Cells of the innate immune system.

There are also many cells and proteins found throughout the body that are part of the innate immune system and are involved in defence against pathogens. Collectively these cells and proteins perform two important functions: they are able to recognise the presence of a foreign entity, for example a bacterial infection, and they provide an immediate cellular response to the presence of an infectious agent. The nature of this cellular response differs according to the nature of the cell and the way in which it is stimulated.
One important cell type of the innate immune system is the macrophage, which is a bone marrow-derived cell type found in most tissues. Some macrophages reside for a long time in the same tissue and are therefore called fixed macrophages. Other macrophages live up to their name of wandering macrophages by moving through tissue. Macrophages are related to a type of white blood cell called a monocyte, which is also derived from bone marrow. Monocytes can leave the bloodstream and enter tissue, where they differentiate into macrophages. Because of their relationship, macrophages and monocytes are known as cells of the monocyte/macrophage lineage.