**Core Concept**
Leucopenia refers to a reduction in the total number of white blood cells (leukocytes) in the blood. It can be caused by various factors, including bone marrow failure, infection, or medication side effects. The primary cell types affected in leucopenia include neutrophils, lymphocytes, monocytes, and eosinophils.

**Why the Correct Answer is Right**
Leucopenia is often characterized by a decrease in the absolute neutrophil count (ANC). Neutrophils play a crucial role in the body's defense against infections, and a decrease in their count can lead to an increased susceptibility to infections. The bone marrow's ability to produce neutrophils is affected in leucopenia, leading to a reduction in the ANC. This can be due to various factors, including bone marrow failure, chemotherapy, or radiation therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** Lymphocytes are also affected in leucopenia, but they are not the predominant cell type involved. Lymphocytes are a type of white blood cell that plays a key role in the adaptive immune response.

**Option B:** Monocytes are a type of white blood cell that matures into macrophages, which play a role in the innate immune response. While monocytes can be affected in leucopenia, they are not the primary cell type involved.

**Option C:** Eosinophils are a type of white blood cell that plays a role in fighting parasites and in allergic reactions. Eosinophils are not typically the predominant cell type involved in leucopenia.

**Clinical Pearl / High-Yield Fact**
In cases of leucopenia, it is essential to identify the underlying cause and implement appropriate treatment to prevent infections and other complications. Absolute neutrophil count (ANC) is a critical parameter in assessing the severity of leucopenia and guiding treatment decisions.

**Correct Answer:** C.