The interaction of immunological defence mechanisms and inflammation is reviewed. To begin with, those cells are discussed, which play a role in the immunological response, viz. the cells by which antigen is recognized (macrophages, B and T lymphocytes) and the effector cells (macrophages, plasma cells and sensitized T cells) and their products. Then, the cells involved in an inflammatory response (neutrophilic, eosinophilic and basophilic granulocytes, mast cells) are discussed, attention also being paid to their products and functions. The fact is stressed that, initially, there is an inflammatory response, resulting in the formation of antigen-antibody complexes or interaction of antigens and lymphokines of sensitized T cells, inflammatory cells being subsequently attracted. Finally, a few instances are cited in support of the theory that both immunological defence mechanisms and the inflammatory response are important factors in the defences of the host against invading micro-organisms, parasites and tumour cells.