\begin{tabular}{l} A student in the classroom has tested positive for \\ COVID and has been asked to quarantine for two \\ weeks. The student tested positive again for the same \\ virus three months after returning to school. \\ Explain in a 300-word essay how his innate and \\ adaptive immune response reacted at the first \\ encounter with the virus. Make sure to include all cells, \\ tissue, and cytokines ... that you learned in this \\ chapter. Then explain how the body responds \\ differently to 2nd encounter of the same virus. \\ Hint: think about memory cells. \\ \hline\end{tabular}

A student's initial encounter with the virus engaged both the innate and adaptive arms of the immune response. Upon invasion, the virus is first detected by cells of the innate immune system. Pattern recognition receptors (PRRs) on macrophages and dendritic cells recognize conserved viral components, leading to rapid activation of these cells. Macrophages engulf the virus and secrete cytokines such as interleukin \( \text{IL-1} \), interleukin \( \text{IL-6} \), and tumor necrosis factor \( \text{TNF-}\alpha \) to initiate inflammation. Natural killer (NK) cells are also alerted and proceed to eliminate virus-infected cells. Dendritic cells process viral antigens and travel to nearby lymph nodes where they present antigen fragments to naive T cells, marking the transition to the adaptive response. In the adaptive immune system, CD4\(^+\) T helper cells are activated and secrete additional cytokines like interleukin \( \text{IL-2} \) that stimulate the proliferation of other immune cells. Cytotoxic CD8\(^+\) T cells are directed to destroy infected host cells presenting viral peptides via major histocompatibility complex (MHC) class I molecules. Concurrently, B cells recognize free viral antigens and become activated with the help of T helper cells. This prompts B cells to differentiate into plasma cells that secrete specific antibodies (initially of the IgM isotype, followed by class switching to IgG) which neutralize the virus and prevent further spread. After the primary infection resolves, a pool of memory B and T cells is generated. These long-lived memory cells persist and rapidly respond upon re-exposure to the same pathogen. On the second encounter, when the student tests positive again, the adaptive immune system utilizes these memory cells for a swift and potent response. Memory B cells promptly differentiate into plasma cells that produce high-affinity IgG antibodies, while memory T cells efficiently target infected cells. This secondary response is faster and more vigorous than the primary response, typically reducing disease severity and lowering viral load. In summary, the primary encounter sets the stage for long-term immunological memory that enables a more rapid and effective reaction upon subsequent exposure to the virus.