Immunology: Eosinophils and Basophils

Eosinophils

• Eosinophils are white blood cells that contain secondary granules rich in major basic protein.
• These granules help to destroy parasites and play a role in immediate hypersensitivity reactions.
• Eosinophilia, an increased number of eosinophils, can be caused by neoplastic, reactive, or idiopathic factors.
• Parasitic infections are the most common cause worldwide, while allergic disorders are most common in developed countries.

Basophils

• Basophils are white blood cells that make up 1% to 2% of peripheral blood.
• They are uncommon in isolation but can increase in response to allergy, hypersensitivity, or inflammation.
• This is due to histamine, a major component of basophilic granules.
• Basophilia can also indicate a malignant process, particularly in myeloproliferative neoplasms, where the count is significantly higher due to elevated white blood cell count.
• Chronic myelogenous leukemia (CML) is the most common cause of basophilia.

Monocytes

• Monocytes constitute 2% to 9% of peripheral blood.
• They function in phagocytosis, antimicrobial activities, tissue repair, and other functions after migrating to tissue.
• These cells contain fine granules which contribute to the destruction of microbes.

Neutrophils

• Neutrophils are the most numerous leukocytes, accounting for 50% to 70% of circulating white blood cells in adults.
• They act as phagocytes capable of ameboid movement into tissues to engulf and destroy bacteria or fungi, making them the first responders to infection.
• They also mediate inflammatory processes.
• Mature neutrophils are smaller than myeloid precursors, enhancing their mobility and deformability.
• Neutrophils exist in two pools: marginating and circulating.
• The marginating pool, not measured by peripheral blood sampling, consists of cells adhering to vessel endothelium within the vascular spaces.
• They enter tissues by diapedesis in response to inflammation or injury.
• Neutrophils in the circulating pool leave the blood by random migration after a half-life of ~7 hours.
• These neutrophils do not return to the bloodstream from tissues.
• After entering tissues, neutrophils are believed to remain for several days, eventually dying by apoptosis or being destroyed by phagocytic cells.
• Neutrophil function during phagocytosis occurs in three phases: migration and diapedesis; opsonization and recognition; and ingestion, killing, and digestion.
• Neutrophils in the marginating pool move along vessel endothelium randomly, a process called locomotion.
• This movement becomes directional in response to chemoattractants, known as chemotaxis.
• Chemoattractants can accelerate neutrophil migration, a phenomenon called chemokinesis.
• Neutropenia can be acquired secondary to aplastic anemia, malignancy of the bone marrow, or dietary deficiencies in B12 or folate.
• Congenital neutropenias, rare disorders, are characterized by defective or deficient neutrophil production in the bone marrow.
• Qualitative neutrophil disorders involve hereditary abnormalities in function, leading to bacterial infections.
• Functional defects may be acquired or inherited, classified by their type: phagocytic/killing defects, granule structure and function defects, defects affecting chemotaxis and motility, and adhesion defects.
• Chediak–Higashi syndrome, a rare neutrophil disorder, is characterized by recurring bacterial infections, partial albinism, and the presence of giant lysosomal granules in nucleated cells.
• Management involves prophylactic antimicrobial therapy and aggressive intravenous treatment during infection.

Lymphocytes

• Lymphocytosis indicates an increased number of lymphocytes in the peripheral blood.
• In adults, lymphocytes represent 20% to 40% of the relative differential count.
• Relative lymphocytosis refers to an increased percentage of lymphocytes in a white blood cell differential.
• Absolute lymphocytosis refers to the total number of lymphocytes present.
• Reactive lymphocytes are transformed or benign lymphocytes.
• Reactive lymphocytes and normal lymphocytes vary in size, shape, and immunophenotypic markers.
• Malignant lymphocytes are cancerous lymphocytes.

Monocytes (Continued)

• Monocytes secrete proteins that regulate hematopoiesis, stimulate inflammatory reactions, remove infectious organisms, senescent blood cells, and modulates the immune function.
• They also stimulate host defense against tumor cells.
• Reactive monocytosis is associated with chronic infections, autoimmune diseases, splenectomy, and various malignancies.
• Absolute monocytosis is found in malignant hematopoietic disorders such as chronic myelomonocytic leukemia (CMML), chronic myelogenous leukemia (CML), acute monocytic leukemia, and acute myelomonocytic leukemia (AMML).

This quiz explores the characteristics and functions of eosinophils and basophils, two types of white blood cells. You will learn about their roles in immune responses, associated conditions, and their relevance in allergic and parasitic reactions. Test your knowledge on these immune cells and their importance in health and disease.