The T and NK cells in patients with primary, or familial, HLH can’t kill virus-infected or other abnormal cells in the patient’s body like they normally would. T cells and NK cells normally do this by secreting death signals into targeted abnormal cells.
The proteins made by the MUNC 13-4, STXBP2, STX11, Rab27a and LYST genes work like the machinery of a conveyor belt, and are responsible for the secretion of the death signals produced by T and NK cells. The PRF1 gene makes a protein called perforin. It works like a key that allows the secreted death signals to enter inside a targeted abnormal cell, where the death signals can work.
SH2D1A is responsible for a more specialized mechanism of killing, and also controls how the T cells themselves die. It is not yet entirely clear why XIAP / BIRC4 mutations cause HLH.