Table 3.

Outcome of selections within thymus and testis

Feature	Type of selection
	T cell selection in the thymus	Sperm receptor selection in the testis
Positive selection on MHC molecules	T cells with low affinity for self-MHC survive	Probably not necessary
Negative selection with self-molecules on/in cells with ‘promiscuous’ transcription	T cells lacking high affinity to self-MHC:peptide complexes survive	Spermatozoa are fitted with receptors that lack self-reactivity
Consequence	T cells recognize nonself-MHC:peptide complexes	Spermatozoa recognize nonself-ligands