Table 3 summarizes the most statistically significant results (p ≤ 0.02) from the comparison of HLA allele and two-loci haplotype frequencies between the two patient groups (R vs. S group). Alleles and haplotypes with a frequency > 2% were considered. Notably, there was a significantly lower frequency of the HLA-DRB1*04:05:01 allele in the R group patients affected by the rapid progression form of IPF compared to S group patients [2/68 (2.94%) vs. 20/138 (14.49%); OR 0.18 (95% CI 0.02–0.78), p = 0.014]. Here, HLA-DRB1 is linked to idiopathic pulmonary fibrosis.