antibody binds to the antigen on the cell surface, whereas in type III the antibody binds to
soluble antigen that was released into the blood or body fluids, and the complex is then
deposited in the tissues.
Immune complexes can be of various sizes, depending on the relative amounts of antigen and
antibody. Fairly large immune complexes are cleared rapidly from the circulation by tissue
macrophages, whereas very small complexes eventually are filtered from blood through the
kidneys, without any pathologic consequences. Intermediate-sized immune complexes are likely
to be deposited in certain target tissues, where they have severe pathologic consequences, such
as inflammation in the kidneys (glomerulonephritis), the vessels (vasculitis), or the joints
(arthritis or degenerative joint disease).
Raynaud phenomenon, a condition caused by the temperature-dependent deposition
of immune complexes in the capillary beds of the peripheral circulation (blocking
Certain immune complexes precipitate at temperatures below normal body
temperature, particularly in the tips of the fingers, toes, and nose, and are called cryoglobulins.
The precipitates block the circulation and cause localized pallor and numbness, followed by
cyanosis (a bluish tinge resulting from oxygen deprivation) and eventually gangrene if the
circulation is not restored.
Type IV hypersensitivity reactions.
Whereas types I, II, and III hypersensitivity reactions are mediated by antibody, type IV reactions
are mediated by T lymphocytes and do not involve antibodies.
Clinical examples of type IV hypersensitivity reactions include graft rejection and allergic
reactions resulting from contact with such substances as poison ivy and metals. Intradermal
injection screening for TB is a delayed response Type IV. Also indicated in in many autoimmune
Autoimmune and Alloimmune Diseases
Many examples of autoimmune or alloimmune diseases have been described. Several basic
principles are exemplified by two examples: systemic lupus erythematosus (an autoimmune
disease) and tissue rejection (i.e., transplant rejection or transfusion reaction) (an alloimmune
Systemic Lupus Erythematosus
Systemic lupus erythematosus (SLE) is a chronic, multisystem, inflammatory disease and is one
of the most common, complex, and serious of the autoimmune disorders. SLE is characterized