Correct option is D
The correct answer is activation of natural killer (NK) cells of the innate immune system. NK cells are unique among immune cells because their activation depends on the recognition (or absence) of self-molecules, particularly self-MHC class I molecules. This recognition is crucial to distinguish healthy self-cells from infected or transformed (e.g., tumor) cells.
Information Booster:
- NK cells are part of the innate immune system, capable of killing abnormal cells without prior sensitization.
- Self-MHC I recognition by inhibitory NK receptors is a key determinant of NK cell activity.
- NK cells follow the "missing-self hypothesis": lack of self-MHC I leads to cell killing.
- NK cells also express activating receptors (e.g., NKG2D) that respond to stress-induced ligands.
- Proper education or licensing of NK cells during development depends on interaction with self-MHC.
- This system helps prevent autoimmunity while maintaining rapid defense against infected or transformed cells.
- NK cell-mediated cytotoxicity involves perforin-granzyme pathway and ADCC (antibody-dependent cellular cytotoxicity).
Additional Information: Incorrect Options
- (a) B cell activation and antibody production: Requires recognition of foreign antigens, not self-molecules.
- (b) Hematopoietic stem cell differentiation: Governed by growth factors and transcriptional cues, not self-recognition mechanisms.
- (c) T cell receptor recombination: Initiated by genetic rearrangement (RAG enzymes), not regulated by self-molecule recognition — although self-recognition is involved later in selection in the thymus, not recombination itself.
