## **Core Concept**
Infectious mononucleosis, caused by Epstein-Barr virus (EBV), is characterized by the presence of atypical lymphocytes in the peripheral blood. These atypical lymphocytes are a hallmark of the disease and play a crucial role in its diagnosis. The condition primarily affects B lymphocytes.

## **Why the Correct Answer is Right**
The atypical lymphocytes seen in infectious mononucleosis are primarily **activated B lymphocytes** and **suppressor T lymphocytes**. However, the hallmark cells, often referred to as "atypical lymphocytes," are a mix but predominantly consist of **T cells**, specifically **CD8+ T cells**, which are cytotoxic T cells that react to the EBV-infected B cells. These T cells are expanded in response to the infection and can appear atypical under the microscope.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While B lymphocytes are indeed involved in infectious mononucleosis as they are the primary target for EBV infection, the atypical lymphocytes predominantly observed are not B cells but rather T cells.
- **Option B:** This option might seem plausible because B cells are infected by EBV, but the atypical lymphocytes are mainly T cells.
- **Option C:** This option could potentially be confusing because plasma cells (mature B cells) can be seen in various infections, but they are not the primary atypical lymphocytes observed in infectious mononucleosis.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the presence of atypical lymphocytes in the blood, along with symptoms such as fever, sore throat, and lymphadenopathy, is highly suggestive of infectious mononucleosis. The **heterophile antibody test (Monospot test)** is often used for rapid diagnosis, though it is not specific for EBV.

## **Correct Answer:** D. T cells.