**Question:** Antigenic variation is seen in all EXCEPT:

A. Eukaryotic cells
B. Prokaryotic cells
C. Viruses
D. Bacteria

**Core Concept:** Antigenic variation is a strategy utilized by certain microorganisms, particularly viruses, to evade the host's immune response by modifying their surface antigens. This allows them to persist within the host and avoid destruction by the host's immune system.

**Why the Correct Answer is Right:** Antigenic variation is primarily seen in microorganisms like viruses, which can change their surface proteins (antigens) to avoid detection by the host's immune system. This mechanism is particularly relevant for viruses, as they are obligate intracellular pathogens and cannot alter other parts of their genome to evade the immune response.

**Why Each Wrong Option is Incorrect:**

A. Eukaryotic cells, including host cells, do not exhibit antigenic variation as they have a stable genome and do not need to evade the immune system.
B. Prokaryotic cells, like bacteria, also do not exhibit antigenic variation since they have a stable genome and do not face the same immune pressure as viruses.
C. Eukaryotic viruses, like herpesviruses, can undergo antigenic variation through a process called latency-reactivation, where they can switch between a latent and replicating state, changing their antigenic profile.
D. Bacterial pathogens, like Streptococcus pneumoniae, can change their surface proteins (antigens) as a form of antigenic variation, allowing them to avoid host immune response.

**Clinical Pearl:** Antigenic variation is a crucial concept in understanding the immune evasion strategies utilized by microorganisms, particularly viruses, to persist within the host and avoid destruction by the immune system. This knowledge is essential for understanding infectious diseases, designing therapeutic strategies, and developing vaccines.