Heat exchanger

The invention relates to a heat exchanger (1) for exchanging heat between a first fluid and a second fluid, comprising a block (2) for conducting the two fluids separately and so as to exchange heat, said block having a number of first flat tubes (3) through which the first fluid flows and which comprise a narrow tube side (4) and a wide tube side (5), and comprising a box (8) which has a base (7), said flat tubes (3) being guided into the box (8) via corresponding through-openings (6) in the base (7). Each through-opening (6) has at least one raised edge (9) which surrounds the corresponding flat tube (3). The wide edge sides (10) are higher than the narrow edge sides (11) and transition into one another via a corresponding inclination (12). It is essential to the invention that a recess (13) is provided in the region of the inclination (12), the height (h1) of said recess being lower than the height (h2) of the narrow edge side (11), and a contact surface edge (14) of a contact surface (16) between the edge (9) and the flat tube (3) has a lower height (h5) in the region of the recess (13) than in the region of the narrow edge side (11). In this manner, a stress load can be reduced and the service life can be extended.