Neutrophils or PMNs are generally the first cells to migrate to the site of an invading microorganism or the site of trauma. This directed migration (chemotaxis) is caused by the release of signaling molecules called chemokines which can be released by several different cell types at the site of inflammation. The PMNs eliminate invaders by phagocytosis and other mechanisms. PMNs comprise 50-70% of the circulating leukocytes and more than 90% of the circulating granulocytes.

PMN's have three types of granules:
  1. The primary or azurophilic granules are lysosomes that contain powerful digestive enzymes including acid hydrolases, elastase, myeloperoxidase and other proteins such as lysozyme and defensins.
  2. Secondary or specific granules contain lactoferrin, lysozyme, collagenase and other proteins.
  3. Tertiary granules contain gelatinase and other enzymes.
Image: Diagram of a PolyMorphoNuclear leukocyte cell.

Eosinophils are involved in defense against parasitic infections and in control of allergic (hypersensitivity) reactions. Eosinophils comprise 1% to 3% of blood leukocytes.

Circulating basophils comprise less than 1% of leukocytes. Granules in basophils contain heparin, histamine, and serotonin. When these (and other) chemicals are released from the cell, they cause an acute inflammatory response, which is why they are collectively called mediators of inflammation. Basophils are related to mast cells, which are found in the tissues only. Mast cells and basophils are the cells involved in immediate hypersensitivity (Type I) reactions (anaphylaxis).