What function of the CF allele protects a carrier from contracting typhoid?

The mutated cystic fibrosis transmembrane conductance regulator (CFTR) blocks the entry of Salmonella typhi into the epithelial cells.

Explanation:

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR):

CFTR is a membrane protein and a chloride channel that regulates the influx and eflux of chloride ions. Cystic fibrosis is a disorder that occurs due to the presence of two recessive alleles in an individual.

Background:

In terms of immunology and defense against infectious disease, CFTR acts as a receptor for the bacteria, Pseudomonas aeruginosa, which is the leading cause of infections in CF patients. P. aeruginosa binds to normal CFTR receptors and is effectively removed from the body. But a mutated CFTR cannot bind the bacteria effectively, leading to life long P. aeruginosa infections.

Mutated CFTR Immune to Salmonella typhi:

Salmonella typhi enters the body by binding to the CFTR receptor. As a natural defense mechanism, S. typhi binds to CFTR on the epithelial cells of the GIT, which, in turn, engulf the bacteria and then slough off from the lining. But, a severe attack leaves the lining open for attack as the epithelial layer sloughs off.

A mutated or missing CFTR receptor is unable to effectively bind to S. typhi; leaving the carrier of the mutated CF allele immune to typhoid infections.