The influence of variously processed feedstuffs on the gastrointestinal tract, various blood parameters, and T cell subsets of different localizations was investigated in 36 broilers. Birds were fed a similarly composed feed mixture differing in particle size (coarse vs. fine). Feedstuffs with different particle size distribution were additionally hydrothermally treated (HTT) (without-non-compacted, pelleted, expanded, and pelleted). The HTT affected BW at slaughtering day and feed intake (P < 0.001). Birds fed non-compacted feed consumed less feed and were lighter than animals of other feeding groups irrespective of grinding fineness. Proventricular size was influenced by HTT (P < 0.001) but not the weight. Generally, feeding of pelleted feed increased proventricular size. Gizzard weight and length was affected by particle size and HTT (P < 0.01). Non-compacted feed resulted in heavier but shorter gizzards compared to other feeds except for finely ground expanded and pelleted feed. Organ traits of gizzard and proventriculus were strongly related to the feed intake. The jejunum was heavier in birds fed expanded and pelleted feed compared to animals fed solely pelleted feed (P = 0.030). The intestinal segment weight to length ratio (WL ratio) of duodenum, jejunum, and total small intestine was affected by HTT (P < 0.01). Generally, non-compacted feed resulted in lowest WL ratios regardless of the particle size distribution. Feeding variously processed feedstuffs also affected white blood cells, blood sodium, and chloride concentrations (P < 0.05) and tended to influence blood carbon dioxide partial pressure and blood total carbon dioxide levels (P < 0.1) in venous blood. Subsets of CD4/CD8 double negative T cells of caecal tonsils were influenced by HTT (P < 0.043). Non-compacted feed tended to increase these subsets in this localization compared to other feeds. In conclusion, the present study shows that an increased feed intake provoked by feed compaction evoke proventricular and ventricular alterations. Furthermore, feeding of variously processed feedstuffs influences the acid base homeostasis and the local immune system of caecal tonsils.