Coombs' reaction is an
immunological lab test to detect
antibodies which attack the
red blood cells of a person's own body (these are called autoantibodies). The test, which is also known as Coombs Test or an anti
globulin test, is important in the diagnosis of certain
blood disorders.
The test first reacts red blood cells with serum from the test individual, then with antibodies to the autoantibodies. If the autoantibodies exist, they will bind to the red blood cells. When the antibodies to the autoantibodies are added, they will bind the autoantibodies (still attached to the red blood cells) and cause the red blood cells to clump together. Large clumps of red blood cells indicate that the autoantibodies exist; absence of the clumps indicates that they don't.
Some or all of the information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and believe I wrote the entry this writeup is partly or wholly based upon.