The invention relates to a heat exchanger comprising a block of first and second flow channels arranged adjacently to one another, said block being designed to be open at one inflow side and at one outflow side of the first flow channels for the inflow and outflow of a first fluid into or out of said first flow channels, and the second flow channels comprising openings for the inflow and outflow of a second fluid, said block consisting of a first element and a second element, each of these forming second flow channels and a side wall, and these elements being joined together such that the two side walls form block side walls which lie opposite one another, said second flow channels extending between these side walls and forming first flow channels between themselves and the side walls.