Harmful Immunity: A General Scheme
So far we have been considering the successful side of the immune system – its defence role against microbial infection (bottom left). The effectiveness of this is due to two main features: (i) the wide range of pathogens it can specifically recognize and remember, and (ii) the strong non-specific mechanisms it can mobilize to eliminate them.
Unfortunately, both of these abilities can also operate against their possessor. 1 Wide-ranging specificity necessitates an efficient mechanism for avoiding action against ‘self’ determinants (the problem of autoimmunity; centre). Also there are cases where the elimination of non-self material may not be desirable (the problem of transplant rejection; top). 2 Strong non-specific weapons (e.g. complement, polymorphs, macrophages and other inflammatory agents; centre) cannot always be trained precisely on the proper target, but may spill over to damage neighbouring tissues (the problem of hypersensitivity: right).
The nomenclature of these immunopathological reactions has never been very tidy. Originally, any evidence of altered reactivity to an antigen following prior contact was called ‘allergy’, while ‘hypersensitivity’ was defined as ‘acute’, ‘immediate’ or ‘delayed’ on the basis of the time taken for changes – often quite harmless skin test reactions – to appear. In fact ‘harmful immunity’ can arise as a result of inappropriate or excessive responses to foreign antigens (innocuous ones as in many common allergies and allogeneic transplants or as a by-product of the response to pathogens) or to self antigens (giving rise to autoimmunity). In all these cases the basic mechanisms are often shared and can be usefully classified according to the very influential scheme of Gell and Coombs (extreme right). However, this classification only covers hypersensitivities involving adaptive immunity, and it is becoming increasingly clear that many of the most common degenerative diseases, such as atherosclerosis and Alzheimer’s disease are caused by chronic activation of innate immunity, especially macrophages, independently of adaptive immunity. A modified classification that includes ‘innate hypersensitivities’ is therefore probably needed.
TH Helper T cell, which by the recognition of carrier determinants permits antibody responses by B cells and the activation of macro- phages. T cells recognizing self antigens probably exist in every person but are normally kept in check by a variety of mechanisms (see Figs 22 and 38).
B B lymphocyte, the potential antibody-forming cell. B lymphocytes that recognize many, although probably not all, ‘self’ determinants are found in normal animals; they can be switched on to make autoantibody by ‘part-self’ (or ‘cross-reacting’) antigens if a helper T cell can recognize a ‘non-self’ determinant on the same antigen (e.g. a drug or a virus; for further details see Fig. 38).
T C Cytotoxic T cells against ‘self’ cells have been demonstrated in some autoimmune diseases (e.g. Hashimoto’s thyroiditis).
Mast cell A tissue cell with basophilic granules containing vasoactive amines, etc., which can be released following interaction of antigen with passively acquired surface antibody (IgE), resulting in rapid inflammation – local (‘allergy’) or systemic (‘anaphylaxis’) (see Fig. 35).
Complexes Combination with antigen is, of course, the basis of all effects of antibody. When there is excess formation of antibody– antigen complexes, some of these settle out of the blood onto the walls of the blood vessels (especially in the skin and kidneys). Tissue damage may then occur from the activation of complement, PMN or platelets (see Fig. 36). Platelet aggregation is a prominent feature of kidney graft rejection. Alternatively, antibodies can form complexes with self antigens on the surface of cells (type II hypersensitivity), activating complement and damaging tissue.
Complement is responsible for many of the tissue-damaging effects of antigen–antibody interactions, as well as their useful function against microorganisms. The inflammatory effects are mostly due to the anaphylatoxins (C3a and C5a) which act on mast cells, while opsonization (by C3b) and lysis (by C5–9) are important in the destruction of transplanted cells and (via autoantibody) of autoantigens.
PMN Polymorphonuclear leucocytes are attracted rapidly to sites of inflammation by complement-mediated chemotaxis, where they phagocytose antigen–antibody complexes; their lysosomal enzymes can cause tissue destruction, as in the classic Arthus reaction. Paradoxically, impaired function of these cells such as occurs in chronic granulomatous disease and perhaps also Crohn’s disease may lead to chronic bacterial infections becoming established, which in turn lead to chronic inflammation and tissue damage.
MAC Macrophages are important in phagocytosis, but may also be attracted to and activated at the site of antigen persistence, resulting in both tissue necrosis and granuloma formation (see Fig. 37). The slower arrival of monocytes and macrophages in the skin following antigen injection gave rise to the name ‘delayed hypersensitivity’. Bacterial lipopolysaccharide (LPS) and several other microbial mol- ecules can activate macrophages directly, causing TNF and IL-1 release. When this occurs on a large scale, it can result in vascular collapse and damage to several organs. This ‘endotoxin shock’ (a type of hypersensitivity of ‘innate’ immunity) is a feature of infections with meningococci and other Gram-negative bacteria (see Fig. 29). LPS can also directly activate the complement (alternative) and clotting pathways. Macrophages can also be activated by some non-infectious stimuli. Uric acid crystals activate macrophage IL-1 secretion and give rise to the painful symptoms of gout. Chronic macrophage activation by oxidized lipoproteins in blood vessels or the β amyloid protein in brain may underly atherosclerosis and Alzheimer’s disease, respectively.
Types of hypersensitivity (Gell and Coombs’ classification)
1 Acute (allergic; anaphylactic; immediate; reaginic): mediated by IgE antibody together with mast cells (e.g. hay fever). Can also give rise to eosinophil activation, most notably in asthma.
2 Antibody mediated (cytotoxic): mediated by IgG or IgM together with complement or phagocytic cells (e.g. blood transfusion reactions, rheumatic fever, many autoimmune diseases).
3 Antigen–antibody complex mediated: inflammation involving complement, polymorphs, etc. (e.g. Arthus reaction, serum sickness, SLE, chronic glomerulonephritis).
4 Cell mediated (delayed; tuberculin-type): T-cell dependent recruitment of macrophages, eosinophils, etc. (e.g. tuberculoid leprosy, schis- tosomal cirrhosis, viral skin rashes, skin graft rejection).
5 Stimulatory: a proposal to split off from type II those cases where antibody directly stimulates a cell function (e.g. stimulation of the thyroid TSH receptor in thyrotoxicosis).