Autoimmune hemolytic anemia

Principles to understand 3

• IgM and IgG antibodies agglutinate RBC at different temperatures

  • it takes one IgM pentamer to agglutinate red cells, but…

  • it takes a few hundred IgG dimers for the chance that one of them might accidentally stick two red cells together. but IgG binds very tightly, so once the accidental binding occurs, it stays

  • on the other hand, IgM normally have very low affinity and they depend on not having everything jostling very much to stay bound to RBC

  • so, when you cool things down, the IgM antibody can hold on to things better than it can when you heat them up

  • IgG depends on excess jiggling from high temperatures to ensure many opportunities to find the right angle to bind. once it does bind, the increased jiggling at these high temperatures doesn’t knock off the RBC because the binding is so tight

• complement C3 binds covalently to the RBC whereas IgM does not. after IgM falls off of the RBC, C3 still stays on

  • IgM and C3 binds to RBCs in cold areas, e.g. the periphery (fingertips)

  • when the blood returns to the thorax, where it is warmer, IgM falls off, but C3 does not

  • therefore you need to do a direct Coombs test to determine whether there is bound complement (not bound antibody)

• Mycoplasma interacts with I antigen on RBC and breaks tolerance. this is a carbohydrate antigen, which means that it does not require T cell help to induce an IgM response. there is no class switching involved

Category: Pathology Notes