Cancer immunology still has many paradoxical questions. One is of the role of the spleen in oncogenesis as well as its own structural reorganization effect on antitumor immunity. Literary data is contradictory and often radically opposite. Therefore, we conducted organometric studies and found out that at the time of pathohistological confirmation of adenocarcinoma in situ in the colon the spleen significantly decreases in size and loses weight. Histologically, deep destructive and degenerative changes in all components of the organ were found: fibrosis, degeneration and destruction of lymphoid tissue, focal zones of destruction in the red pulp. Submicroscopic analysis revealed the presence of lymphocytes with signs of apoptosis in the mantle and periarterial zones. In the white pulp, changes in part of the plasmatic cells were observed: a decrease in the size of the nuclei, damaged GER tubules and mitochondrial cristae. The severity of structural disorders of the spleen increases in the dynamics of the neoplastic lesion development.