## **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 correct answer, **B cells**, is right because infectious mononucleosis is caused by Epstein-Barr virus, which specifically infects B lymphocytes. The atypical lymphocytes seen in the peripheral blood smear of patients with infectious mononucleosis are primarily activated B cells or lymphoblasts that have been transformed by the virus. These cells can exhibit various morphological abnormalities, including large size, abundant cytoplasm, and irregular nuclear shape.

## **Why Each Wrong Option is Incorrect**
- **Option A:** T cells - While T cells are involved in the immune response to EBV and can be activated, the atypical lymphocytes characteristic of infectious mononucleosis are primarily of B cell origin.
- **Option C:** Natural Killer cells - These cells are part of the innate immune system and are not specifically associated with the atypical lymphocytosis seen in infectious mononucleosis.
- **Option D:** Plasma cells - Although plasma cells can be present in the context of various infections and immune responses, the atypical lymphocytes in infectious mononucleosis are specifically related to B cells.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the presence of atypical lymphocytes in a patient with fever, sore throat, lymphadenopathy, and splenomegaly is highly suggestive of infectious mononucleosis. The diagnosis can be confirmed by the presence of heterophile antibodies (as detected by the Monospot test) and/or by EBV-specific serology.

## **Correct Answer:** . B cells