Basophils are involved in inflammatory reactions including allergic diseases, anaphylaxis, and asthma. (Share many features with mast cells found in connective tissue.)

A basophil has a bilobed nucleus and contains distinctive granules.

• Secretory Granules (orange) - numerous large, round (electron-dense) granules often with myelin figures.
• Histamine - dialates blood vessels and contracts smooth muscle.
• Heparin - reduces blood clotting (responsible for metachromatic staining).
• Cytokines, chemokines, and growth factors. (Some attract eosinophils to the site of inflammation.)
• Release their contents by compund exocytosis or piecemeal degranulation.
• Small Granules (black) - few small, round (electron-dense) granules (not visible). Secretory lysosomes that contain additonal cell-specific proteins.
• Lysosomal degradative enzymes.

Although a basophil contains numerous granules, it contains relatively few organelles.

• Plasma Membrane (dark green)
• Fc receptors that bind IgE antibodies released from plasma cells.
• Nucleus (blue) / Nuclear Envelope (purple) - usually bilobed or S-shaped.
  • Mast cells in connective tissue have a round nucleus.
• Golgi Apparatus - poorly developed (not visible).
• Mitochondria (red) - small, few in number.
• Endoplasmic Reticulum (cyan) - scattered fragments.
• Centriole (orange) - longitudinal section adjacent to the nucleus.
• Cytoplasm (green)

Granule Secretion

Upon activation basophils release the contents of specific granules by two distinct mechanisms.

• Compound Exocytosis (anaphylactic degranulation) - the entire contents are released by fusion of granules with each other as well as with the plasma membrane.
• Rapid, explosive IgE-triggered process.
• Occurs at sites of allergic responses.
• Piecemeal Degranulation - small (round or curved) transport vesicles are used to transport "bits and pieces" of granules to the plasma membrane.
  • Allows the slow release of granule contents.
  • Progressive emptying leaves empty granules that are otherwise intact.
  • Groups of granules emptied in succession.
  • Most common type of secretion in chronic inflammation.

Basophil Activation

Basophils can be activated by a large number of different molecules.

• Binding of antigens to IgE-bound to Fc receptors on their surface.
• Some cytokines, chemokines, and growth factors.
• Factors of the complement system.
• Pathogens derived from bacteria, viruses, and fungi.

Activated basophils release a variety of substances.

• Preformed molecules stored in specific granules - histamine, heparin, and cytokines.
• Newly synthesized molecules - some stimuli induce their synthesis even in the absence of degranulation.
• Cytokines (IL-4 in particular induces B-cell differentiation and IgE synthesis).
• Leukotriene - synthesized from arachondic acid released from lipid bodies that are not bound by a membrane.
• Leukotriene C4 causes prolonged contraction of smooth muscle.