The direct Coomb's test looks for auto-antibodies to RBC's by checking for IgG bound to RBC's using anti-IgG antibodies. The direct Coomb's test is used to check for immune mediated hemolytic anemias.
The indirect Coomb's test detects allo-antibodies to a donor's RBC's in a patient's serum for Rh-system matching when transfusing blood.