Eosinophils are a type of white blood cell, or leukocyte, with a multifaceted role in the body’s defense system. Their classification is complex, as they exhibit characteristics of both innate and adaptive immunity. This article explores eosinophils and their unique position within the immune response.
What Are Eosinophils?
Eosinophils are a type of granulocyte, a white blood cell with granules in its cytoplasm. These cells originate and mature in the bone marrow, stimulated by signaling molecules like interleukin-5 (IL-5), IL-3, and granulocyte-macrophage colony-stimulating factor (GM-CSF). Once released, they circulate in the bloodstream for 4.5 to 8 hours before migrating into various tissues.
Eosinophils are distinguishable by their bilobed nucleus and large cytoplasmic granules that stain red or orange with acidic dyes like eosin. They make up less than 5% of circulating leukocytes in healthy individuals but are predominantly found in connective tissues, especially in the gastrointestinal and respiratory tracts. These cells possess approximately 200 granules filled with enzymes and proteins, which are released upon activation.
The Immune System’s Two Branches
The body’s defense against foreign invaders is orchestrated by two interconnected branches: the innate and adaptive immune systems. The innate immune system is the body’s immediate, first line of defense. It provides a rapid, non-specific response to a wide range of threats. Components of innate immunity include physical barriers like skin and mucous membranes, as well as cells such as macrophages and natural killer (NK) cells.
The adaptive immune system offers a more specialized and targeted defense. This system develops over time upon exposure to specific pathogens, creating immunological memory for a stronger, faster response upon subsequent encounters. Cells like T cells and B cells are central to adaptive immunity, with B cells producing antibodies that specifically target pathogens. The adaptive response is generally slower to activate than the innate system but provides long-lasting protection.
Eosinophils: More Than Just One Category
Eosinophils demonstrate characteristics of both innate and adaptive immunity, bridging these two branches of the immune system. As innate immune cells, they are equipped for immediate, non-specific responses. Upon activation, eosinophils rapidly degranulate, releasing pre-formed cytotoxic proteins such as major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil-derived neurotoxin (EDN), and eosinophil peroxidase (EPO). These proteins are effective against parasitic infections, particularly helminths (worms), and contribute to the inflammatory response in allergic reactions. Eosinophils can also engulf and destroy pathogens through phagocytosis.
Eosinophils also participate in shaping adaptive immune responses. They are recruited to sites of chronic inflammation and interact with various adaptive immune cells. Eosinophils produce cytokines that influence T cell differentiation and activity, such as IL-4, which promotes the development of T helper 2 (Th2) cells. They are capable of presenting antigens to T cells, activating them and promoting an immune response. Eosinophils also interact with dendritic cells, influencing their maturation and ability to present antigens.
When Eosinophils Go Awry
When eosinophil numbers or activity become abnormal, it can lead to various health issues. An elevated level of eosinophils in the blood, known as eosinophilia, is commonly associated with allergic diseases. Conditions such as asthma, allergic rhinitis, and eczema often involve increased eosinophil counts, where their degranulation can contribute to tissue damage and inflammation. Parasitic infections, particularly those caused by helminths, are another frequent cause of eosinophilia, especially during the larval migration phase through tissues.
Conversely, a lower-than-normal eosinophil count, termed eosinopenia, can also occur. This condition may be observed during acute stress responses, including severe infections like sepsis. The use of certain medications, such as corticosteroids, can also lead to decreased eosinophil levels due to their immune-suppressing effects. While eosinopenia itself often does not require specific treatment, its presence can indicate an underlying condition that warrants further investigation and management.