Learning Objectives
- Identify the morphological characteristics of an eosinophil compared to a lymphocyte.
- Recognize the distinctive granules that define eosinophil function.
- Understand the nuclear configuration of different leukocyte types in a peripheral smear.
- Explain the clinical conditions associated with an increased eosinophil count.
Overview of Eosinophils and Lymphocytes
This high-power microscopic view of normal peripheral blood demonstrates an eosinophil and a lymphocyte. These cells represent different arms of the immune system: the eosinophil is a specialized granulocyte, while the lymphocyte is an agranulocyte essential for adaptive immunity.
Characteristic Features: Eosinophil
- Nucleus: Typically bilobed (two lobes connected by a thin thread), resembling “spectacles.”
- Cytoplasm: Filled with large, uniform, reddish-orange granules. These granules contain basic proteins that stain intensely with acidic dyes like eosin.
- Size: Generally larger than a typical small lymphocyte but similar in size to a neutrophil.
Characteristic Features: Lymphocyte
- Nucleus: Large, round, and densely stained. It occupies the vast majority of the cell area.
- Cytoplasm: Appears as a very thin, pale blue rim around the edge of the nucleus.
- Agranulocytosis: Unlike the eosinophil, the lymphocyte lacks the large, prominent cytoplasmic granules.
Clinical Relevance: Eosinophilia
- Allergic Reactions: An increase in eosinophils is a classic clinical finding in Type I Hypersensitivity reactions, such as asthma, hay fever, and drug allergies.
- Parasitic Infections: Eosinophils are specifically equipped to fight helminthic parasites by releasing toxic granule contents (e.g., major basic protein) onto the parasite’s surface.
- Neoplasia: Eosinophilia can sometimes be seen in certain malignancies, including Hodgkin lymphoma and some types of leukemia.