The bridging of antigen particles by antibody molecules forming large visible aggregates or clumps causes the reaction called agglutination. ... Agglutination is used in many clinical tests to determine the antibody titer of sera, or to identify bacteria. (Cornell University)
When two attracting agents are present in a blood sample, then their joint presences will cause them to be drawn close together. To the visible eye, this drawing close together appears as a clumping or lumping together. When anti-A is present in a blood sample, then the type A red blood cells will be attracted and instantly brought close together resulting in the visible clumping called agglutination.