Type III, or immune-complex, reactions are characterized by tissue damage caused by the activation of complement in response to antigen-antibody (immune) complexes that are deposited in tissues. The classes of antibody involved are the same ones that participate in type II reactions—IgG and IgM—but the mechanism by which tissue damage is brought about is different. The antigen to which the antibody binds is not attached to a cell. Once the antigen-antibody complexes form, they are deposited in various tissues of the body, especially the blood vessels, kidneys, lungs, skin, and joints. Deposition of the immune complexes causes an inflammatory response, which leads to the release of tissue-damaging substances, such as enzymes that destroy tissues locally, and interleukin-1, which, among its other effects, induces fever.

Immune complexes underlie many autoimmune diseases, such as systemic lupus erythematosus (an inflammatory disorder of connective tissue), most types of glomerulonephritis (inflammation of the capillaries of the kidney), and rheumatoid arthritis.

Type III hypersensitivity reactions can be provoked by inhalation of antigens into the lungs. A number of conditions are attributed to this type of antigen exposure, including farmer’s lung, caused by fungal spores from moldy hay; pigeon fancier’s lung, resulting from proteins from powdery pigeon dung; and humidifier fever, caused by normally harmless protozoans that can grow in air-conditioning units and become dispersed in fine droplets in climate-controlled offices. In each case, the person will be sensitized to the antigen—i.e., will have IgG antibodies to the agent circulating in the blood. Inhalation of the antigen will stimulate the reaction and cause chest tightness, fever, and malaise, symptoms that usually pass in a day or two but recur when the individual is reexposed to the antigen. Permanent damage is rare unless individuals are exposed repeatedly. Some occupational diseases of workers who handle cotton, sugarcane, or coffee waste in warm countries have a similar cause, with the sensitizing antigen usually coming from fungi that grow on the waste rather than the waste itself. The effective treatment is, of course, to prevent further exposure.

The type of allergy described in the preceding paragraph was first recognized as serum sickness, a condition that often occurred after animal antiserum had been injected into a patient to destroy diphtheria or tetanus toxins. While still circulating in the blood, the foreign proteins in the antiserum induced antibodies, and some or all of the symptoms described above developed in many subjects. Serum sickness is now rare, but similar symptoms can develop in people sensitive to penicillin or certain other drugs, such as sulfonamides. In such cases the drug combines with the subject’s blood proteins, forming a new antigenic determinant to which antibodies react.

The consequences of antigen-and-antibody interaction within the bloodstream vary according to whether the complexes formed are large, in which case they are usually trapped and removed by macrophages in the liver, spleen, and bone marrow, or small, in which case they remain in the circulation. Large complexes occur when more than enough antibody is present to bind to all the antigen molecules, so that these form aggregates of many antigen molecules cross-linked together by the multiple binding sites of IgG and IgM antibodies. When the ratio of antibody to antigen is enough to form only small complexes, which can nevertheless activate complement, the complexes tend to settle in the narrow capillary vessels of the synovial tissue (the lining of joint cavities), the kidney, the skin, or, less commonly, the brain or the mesentery of the gut. The activation of complement—which leads to increased permeability of the blood vessels, release of histamine, stickiness of platelets, and attraction of granulocytes and macrophages—becomes more important when the antigen-antibody complexes are deposited in blood vessels than when they are deposited in the tissues outside the capillaries. The symptoms, depending on where the damage occurs, are swollen, painful joints, a raised skin rash, nephritis (kidney damage, causing blood proteins and even red blood cells to leak into the urine), diminished blood flow to the brain, or gut spasms.

The formation of troublesome antigen-antibody complexes in the blood can also result from subacute bacterial endocarditis, a chronic infection of damaged heart valves. The infectious agent is often Streptococcus viridans, normally a harmless inhabitant of the mouth. The bacteria in the heart become covered with a layer of fibrin, which protects them from destruction by granulocytes, while they continue to release antigens into the circulation. These can combine with preformed antibodies to form immune complexes that can cause symptoms resembling those of serum sickness. Treatment involves eradication of the heart infection by a prolonged course of antibiotics.