This type of reaction occurs when specific kinds of white blood cells (called T helper cells) respond to antigens and release toxic and inflammatory substances that can damage tissues. Cell-mediated immune reactions can occur in any organ. Treatment usually involves eliminating the offending antigen (if possible) and the use of anti-inflammatory drugs and drugs that suppress the immune system, either alone or in combination.
Granulomatous reactions are masses of scar tissue and white blood cells that accumulate around an area of persistent infection. The feline infectious peritonitis virus and some bacteria and fungi may trigger these reactions in cats. These reactions may be due to long-lasting cell-mediated immune reactions. Although cell-mediated immune responses effectively fight off these infections in most cats, in a few animals the immune response is only partially effective and results in the mass at the site of infection.
Also see professional content regarding Type IV reactions.