**Core Concept**
The peripheral blood contains a mixture of immune cells, including white blood cells (leukocytes) such as neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Accurate detection and quantification of these cells are crucial for diagnosing and monitoring various immune-related disorders.

**Why the Correct Answer is Right**
Flow cytometry (FCM) is a powerful laboratory technique that uses light to detect and quantify various components of blood cells, including their surface markers and intracellular proteins. FCM works by passing a single-cell suspension through a laser, which excites fluorescent dyes attached to specific antibodies that bind to cell surface receptors. This allows for the simultaneous measurement of multiple cell surface antigens, enabling the identification and enumeration of specific immune cell populations. FCM is particularly useful for diagnosing and monitoring hematological malignancies, such as lymphomas and leukemias.

**Why Each Wrong Option is Incorrect**
**Option A:** Complete Blood Count (CBC) is a basic laboratory test that measures various parameters of blood cells, including hemoglobin, hematocrit, and white blood cell count. However, it lacks the specificity and sensitivity of FCM for detecting and quantifying immune cells.
**Option B:** Blood smear examination involves visual inspection of a stained blood smear under a microscope to identify abnormal cells and estimate cell ratios. While useful for initial screening, it is subjective and less accurate than FCM for precise cell counting and subtyping.
**Option C:** Enzyme-linked immunosorbent assay (ELISA) is a laboratory technique used to detect and quantify specific proteins or antigens in blood. However, it is not suitable for detecting and counting immune cells.

**Clinical Pearl / High-Yield Fact**
Flow cytometry is a highly sensitive and specific technique for diagnosing and monitoring hematological malignancies, such as acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). It can also aid in the diagnosis of autoimmune disorders, such as rheumatoid arthritis and lupus, by detecting abnormal immune cell populations.

**Correct Answer:** C. Flow cytometry.