It includes:

• The phagocytic system (white blood cells, including macrophages, neutrophils, and monocytes) - whose function is to ingest and digest invading microorganisms
• Inflammatory mediators produced by various cells, including basophils, mast cells, and eosinophils
• Natural killer (NK) cells that are specialized lymphocytes that kill some tumors cells, microorganisms, and cells that have been infected by viruses
• Acute phase reactants and cytokines, which are a group of soluble proteins that can cause changes in the growth of many cells, including the white blood cells that produce them.