Underdetermination of theories by evidence’ is one traditional approach in antithesis to scientific realism. The rationale of the so-called ‘underdetermination of theories by evidence’ is that at any moment, evidence will always be insufficient to support, entirely, one scientific theory. For any scientific theory, it is possible to find a rival theory that explains empirical evidence equally well. In this paper, I argue that the concept of underdetermination is not applicable in the context of contemporary biomedical sciences (including human immunology). The definition of observable and unobservable should be adapted to technological advances. For example, it is problematic to classify, contemporarily, an antibody as unobservable. Accepting underdetermination without making use of available (and proven reliable) scientific techniques and previous knowledge, is unthinkable. Contemporary biomedical sciences do not allow themselves to be limited by apparent obstacles such as the ‘underdetermination of theories by evidence’. When contemporary biomedical sciences are faced with an impasse, such as apparent rival theories, scientists seek new evidence until solving the underdetermination. In this paper, I provide an example from immunology that undermines the concept of underdetermination in immunology.