Atypical lymphocytes in infectious mononucleosis are made up of –
**Core Concept**
Infectious mononucleosis, caused by Epstein-Barr virus (EBV), is characterized by lymphocytosis, particularly an increase in atypical lymphocytes. These atypical lymphocytes are a result of the immune response to EBV, leading to an abnormal proliferation of lymphocytes.
**Why the Correct Answer is Right**
Atypical lymphocytes in infectious mononucleosis are primarily composed of activated T cells, specifically CD8+ T cells, which recognize and kill virus-infected cells. These T cells are activated by the presentation of viral antigens by infected B cells, leading to their proliferation and differentiation into atypical lymphocytes. The atypical appearance of these lymphocytes is due to their activated state and their expression of surface markers such as HLA-DR and CD25.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while B cells are infected with EBV, they are not the primary component of atypical lymphocytes.
**Option B:** This option is incorrect because natural killer (NK) cells are not the primary component of atypical lymphocytes in infectious mononucleosis.
**Option C:** This option is incorrect because while activated T cells are the primary component of atypical lymphocytes, the specific subset is CD8+ T cells, not CD4+ T cells.
**Clinical Pearl / High-Yield Fact**
It's essential to recognize that atypical lymphocytes are not specific to infectious mononucleosis and can be seen in other viral infections, such as cytomegalovirus (CMV) and human immunodeficiency virus (HIV) infection.
**Correct Answer:** C. CD8+ T cells.