We covered a number of situations in lecture where the immune response is directed against an inappropriate target; we described these as hypersensitivites, where the combination of antigen and either antibodies (Type I, II, or III) or cells (Type IV) led to the develop of pathology and tissue damage. Our in class examples of the Type IV (Delayed Type) Hypersensitivity was (1) the Tuberculin response used to determine reactivity against Mycobacterium tuberculosis and (2) the immune response against urusiol, the antigen found in poison ivy. One fascinating example not discussed in class of Type IV hypersensitivity, but was seen on the Powerpoint slide, is the process that occurs during transplantation of tissue from a donor to a recipient.
When a recipient receives a transplant, the new tissue contains antigens (endogenous antigens) that will be perceived as foreign by the recipient’s immune system. Recall that T cytotoxic cells respond to the presence of endogenous antigens, in contrast to the antibody response against exogenous antigens. These transplant-directed T cytotoxic cells will work to destroy the antigen found on transplanted tissue, as if they were cells infected with a virus, resulting in the rejection of the transplant within a few weeks. The standard method to prevent this process is to place the transplant patient on immune suppression drugs for the remainder of their lives. This will of course render them somewhat more susceptible to opportunistic infections, but that would be a small price to pay for the benefits afforded by transplant.
One special case of Type IV hypersensitivity is Graft versus Host disease, which can be the result of a bone marrow transplant. In a bone marrow transplant, donor immunologic cells are transplanted into a recipient who has received total body irradiation to eliminate their own bone marrow stem cells. The donated cells then hopefully reconstitute functioning bone marrow, and produce directed immune responses when the donor comes into contact with foreign antigens. Sometimes, the donated bone marrow cells may see the recipient’s cells as “foreign” and develop a response against the host cells. Again, immune suppression drugs can be administered to diminish the possibility of Graft versus Host disease.