Heat stress lowers FI (15.2%), BWG (19.2%), and deteriorates FCR (1.91 vs. 1.81) when compared with those kept in the thermoneutral houseHeat stress reduced antibody responses against Newcastle disease and infectious bronchitis virusHeat stress reduces the proportion of helper (CD4+) T lymphocytes and increases cytotoxic (CD8+) T lymphocytes, leading to a lower CD4 + to CD8 + ratio in peripheral blood circulation. This evidence concerns the gene CD4 and Newcastle disease.