Monocytes share the 'vacuum cleaner' function of neutrophils, but are much longer lived as they have an additional role. They present pieces of pathogens to T cells so that the pathogens may be recognized again and killed, or so that an antibody response may be mounted. Monocytes are also known as macrophages after they leave the bloodstream and enter tissue.

Monocytes are responsible for phagocytosis, or digestion, of foreign substances in the body. Monocytes can perform phagocytosis using intermediary (opsonising) proteins such as antibodies or complement that coat the pathogen, as well as by binding to the microbe directly via pathogen-recognition receptors. Monocytes are also capable of killing infected host cells via antibody, termed antibody-mediated cellular cytotoxicity. Vacuolization may be present in a cell that has recently phagocytized foreign matter.

Mast Cells are similar to basophils. Mast cells contain a variety of inflammatory chemicals including histamine and seratonin. They cause blood vessels near wound to constrict and are involved in the inflammatory process. Mast cells are integral in host defense mechanisms. These cells are involved in hypersensitivity reactions, acute and chronic inflammation, T-cell stimulation, and tissue defense against parasites.

Mast cells have a single large nucleus and no granulocytes.

Macrophages (caption above) are involved at all stages of the immune response. They act as a rapid protective mechanism which can respond before T cell-mediated amplification has taken place. Activated macrophages play a key role in host defense against intracellular parasitic bacteria, pathogenic protozoa, fungi and helminths as well as against tumors, especially metastasing tumors. After phagocytosis, macrophages prevent intracellular parasitic organisms from reproducing.

In addition, macrophages are important killer cells (K cells); by means of antibody-dependent cell-mediated cytotoxicity (ADCC) they are able to kill or damage extracellular targets. They also take part in the initiation of T cell activation by processing and presenting antigen. Finally they are central effector and regulatory cells of the inflammatory response. To fulfill these functions, macrophages in their activated state are able to produce more than one hundred of different substances.

Monocytes