Immunopathology

Abstract

Immunopathology (a term made up of 2 words with opposite meanings) includes not only more disease entities than any other specified field but also many basically nonallergic diseases with allergic complications. The tissue alterations in allergic reactions belong to the inflammatory response. Immunopathologic mechanisms may be divided into 6 categories: neutralization or inactivation; cytotoxicity or cytolysis; atopy or anaphylaxis; toxic complexes or the Arthus reactions; delayed or cellular response; and the granulomatous reaction. The first 4 types are due to circulating antibody combining with a given antigen in vivo. Delayed or cellular reactions are not mediated by humoral antibodies but by specifically sensitized cells. Granulomatous reactions are separated from the more classically recognized delayed or cellular reactions because of the pathologic appearance of the tissue reactions. Antibody-mediated diseases are due to circulating antibody combining in vivo with antigens, and have in common: the hypersensitive state, induced by previous exposure to antigen (or passive transfer of antibody); a definite induction or latent period comparable to other immunologic responses (1 to 2 wk); it occurs only on exposure to the specific antigen or to closely related substances (cross reaction); it is not necessarily determined by the characteristics of the antibody or species of animal, i.e., details of anaphylaxis depends on distribution of smooth muscle; the degree of hypersensitivity tends to diminish with time, but reexposure to antigen results in reappearance of hypersensitivity more rapidly and more intensely than primary exposure (secondary or anamnestic response); the hypertensive state can be passively transferred with serum (antibody); administration of antigen with proper precautions to avoid death can result in temporary desensitization, i.e., loss of the ability to react due to saturation of antibody available at the time; while the type of tissue reaction or effect depends on the nature and location of the antigen and on the properties of the antibody. Cellular-mediated diseases are reactions of specifically sensitized lymphocytes with antigen. Such reactions may release a number of mediators which increase the tissue response by recruitment of other mononuclear cells. The major difference between this type of allergic response and those listed above is that delayed hypersensitivity reactions cannot be transferred or initiated by circulating immunoglobulin antibody. In granulomatous hypersensitivity organized collections of altered mononuclear cells are formed, called granulomas. The nature of antigen recognition in granulomatous hypersensitivity remains unknown, but this type of hypersensitivity may be due to delayed reactivity to insoluble antigens.