​Virus-infected cells are cleared by cytolysis by natural killer (NK) cells and CD8⁺ Cytotoxic T lymphocytes (CTLs). Which of the following graphs correctly represents the changes in the cell numbers of the two cell types during the course of a viral infection, considering that the virus is cleared by the 14th day?​

Correct option is A

Explanation-

During a typical acute viral infection, the kinetics of immune cell response are:
1. NK Cells (Natural Killer Cells)
    (i) Part of innate immunity
    (ii) Respond very early (within hours to 1–2 days)
    (iii) Peak quickly, then decline
Function: recognize infected cells with reduced MHC I and kill them
So, in the correct graph, NK cell numbers rise early and drop off before day 14.
2. CD8⁺ CTLs (Cytotoxic T Lymphocytes)
    (i) Part of adaptive immunity
    (ii) Require antigen presentation → activation → clonal expansion
    (iii) Rise later (around day 3–4), peak around day 7–10
Decline after virus is cleared (day 14)
So, CD8⁺ CTLs rise later than NK, peak around day 10, and then decline.
Graph in Option a shows:
   (i) Solid line (NK cells) rises early, peaks, and declines before day 14
   (ii) Dotted line (CD8⁺ CTLs) rises later, peaks near day 10, and then declines
This matches  immunological response pattern.

Incorrect Options -
Option b: CTLs plateau too early and stay high – unrealistic.
Option c: Shows CTLs peaking before NKs – wrong sequence.
Option d: NKs rise after CTLs – again, biologically incorrect.

 Final Answer: Option a is correct.