Cellular Components of the Immune System

Polymorphonuclear Leukocytes (PMNs)

White blood cells with multilobed nuclei and cytoplasmic granules; include neutrophils, eosinophils, and basophils.

Granulocytes

Another name for polymorphonuclear leukocytes because they contain cytoplasmic granules.

Neutrophils

The most abundant circulating white blood cells (40–70%); first responders against infection that phagocytose and destroy pathogens.

Chemotaxis

Directed migration of immune cells toward chemical signals released during infection or inflammation.

Phagocytosis

Process by which cells engulf and digest microorganisms or other foreign particles.

Reactive Oxygen Compounds

Antimicrobial molecules produced by neutrophils during pathogen killing, including superoxide and hypochlorous acid.

Defensins

Antimicrobial peptides released from neutrophil granules that help kill microbes.

Lactoferrin

Iron-binding antimicrobial protein found in neutrophil granules that inhibits microbial growth.

Lysozyme

Enzyme released by neutrophils that damages bacterial cell walls.

Neutrophil Extracellular Traps (NETs)

Web-like structures composed of DNA, histones, and granule proteins that trap and kill microbes.

Chronic Granulomatous Disease

Immunodeficiency disorder in which phagocytes cannot effectively kill pathogens, leading to recurrent bacterial and fungal infections.

Eosinophils

Granulocytes that constitute up to 5% of circulating white blood cells and are specialized for defense against parasites and involvement in allergic responses.

Major Basic Protein

Toxic eosinophil granule protein that damages parasites.

Eosinophil Cationic Protein

Eosinophil-derived protein with cytotoxic activity against parasites, bacteria, and viruses.

Eosinophilia

Increased eosinophil count commonly associated with parasitic infections, allergic diseases, and some cancers.

Eosinophilic Asthma

Asthma subtype characterized by eosinophil-driven airway inflammation.

Eosinophilic Granulomatosis with Polyangiitis

Inflammatory vasculitic disorder associated with eosinophilia and allergic disease.

Eosinophilic Esophagitis

Chronic inflammatory disease of the esophagus characterized by eosinophil infiltration.

Hypereosinophilic Syndrome

Disorder characterized by persistently elevated eosinophil levels causing tissue damage.

Basophils

Granulocytes comprising less than 5% of circulating white blood cells that participate in allergic and inflammatory reactions.

Mast Cells

Tissue-resident immune cells that share functional similarities with basophils, including IgE-mediated activation.

FcεRI (High-Affinity IgE Receptor)

Receptor on basophils and mast cells that binds IgE and initiates allergic responses when cross-linked by antigen.

Degranulation

Release of preformed inflammatory mediators from basophils or mast cells following activation.

Histamine

Inflammatory mediator released during basophil and mast cell degranulation that promotes vasodilation and increased vascular permeability.

Platelet-Activating Factor

Potent inflammatory mediator released by activated basophils and mast cells.

Leukotrienes

Lipid inflammatory mediators synthesized by activated basophils and mast cells.

Prostaglandins

Lipid mediators involved in inflammation and produced after basophil activation.

Thromboxanes

Lipid mediators generated during basophil activation that contribute to inflammatory responses.