Hosts as reactive environments resistance recovery and immunity

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Any reaction by an organism to the presence of another depends on it recognizing a difference between what is 'self' and what is 'not self'. In invertebrates, populations of phagocytic cells are responsible for much of a host's response to invaders, even to inanimate particles. In insects, hemocytes (cells in the hemo-lymph) isolate infective material by a variety of routes, especially encapsulation - responses that are accompanied by the production of a number of soluble compounds in the humoral system that recognize and respond to nonself material, some of which also operate at the midgut barrier in the absence of hemocytes (Siva-Jothy et al., 2001).

Figure 12.3 The immune response. The mechanisms mediating resistance to infection can be divided into 'natural' or 'nonspecific' (left) and 'adaptive' (right), each composed of both cellular elements (lower half) and humoral elements (i.e. free in the serum or body fluids; upper half). The adaptive response begins when the immune system is stimulated by an antigen that is taken up and processed by a macrophage (MAC). The antigen is a part of the parasite, such as a surface molecule. The processed antigen is presented to T and B lymphocytes. T lymphocytes respond by stimulating various clones of cells, some of which are cytotoxic (NK, natural killer cells), as others stimulate B lymphocytes to produce antibodies. The parasite that bears the antigen can now be attacked in a variety of ways. PMN, polymorphonuclear neutrophil. (After Playfair, 1996.)

In vertebrates there is also a phagocytic response to material that is not self, but their armory is considerably extended by a much more elaborate process: the immune response (Figure 12.3). For the ecology of parasites, an immune response has two vital features: (i) it may enable a host to recover from an infection; and (ii) it can give a once-infected host a 'memory' that changes its reaction if the parasite strikes again, i.e. the host has become immune to reinfection. In mammals, the transmission of immunoglobulins to the offspring can sometimes even extend protection to the next generation.

For most viral and bacterial infections of vertebrates, the colonization of the host is a brief and transient episode in the host's life. The parasites multiply within the host and elicit a strong immunological response. By contrast, the immune responses elicited by many of the macroparasites and protozoan microparasites tend to be weaker. The infections themselves, therefore, tend to be persistent, and hosts may be subject to repeated reinfection.

Indeed, responses to microparasites and helminths seem often to be dominated by different pathways within the immune system (MacDonald et al., 2002), and these pathways can down-regulate each other: helminth infection may therefore increase the likelihood of microparasitic infection and vice versa (Behnke et al., 2001). Thus, for example, successful treatment of worm infections in patients that were also infected with HIV led to a significant drop in their HIV viral load (Wolday et al., 2002).

The modular structure of plants, the presence of cell walls and the absence plants of a true circulating system (such as blood or lymph) all make any form of immunological response an inefficient protection. There is no migratory population of phagocytes in plants that can be mobilized to deal with invaders. There is, however, growing evidence that higher plants possess complex systems of defense against parasites. These defenses may be constitutive - physical or biological barriers against invading organisms that are present whether the parasite is present or not - or inducible, arising in response to pathogenic attack (Ryan & Jagendorf, 1995; Ryan et al., 1995). After a plant has survived a pathogenic attack, 'systematic acquired resistance' to subsequent attacks may be elicited from the host. For example, tobacco plants infected on one leaf with tobacco mosaic virus can produce local lesions that restrict the virus infection locally, but the plants then also become resistant to new infections not only by the same virus but to other parasites as well. In some cases the process involves the production of 'elicitins', which have been purified and shown to induce vigorous defense responses by the host (Yu, 1995).

Central to our understanding of all host defensive responses to parasites is the belief that these responses are costly - that energy and material invested in the response must be diverted away from other important bodily functions - and that there must therefore be a trade-off between the response and other aspects of the life history: the more that is invested in one, the less can be invested

Natural ('nonspecific')



Viruses -

Some bacteria







Some bacteria

Some bacteria



Some bacteria

Nonspecific Immunity Lysozyme


Entry block neutralization (toxin)


inflammation I,

Specific antigens

Specific antigens r\ ;

Phagocytosis Cytotoxicity

Phagocytosis Cytotoxicity

Myeloid cells

Cytotoxicity (virus)

(all bacteria viruses, etc.)

Cytotoxicity (virus)


Myeloid cells

Lymphocytes vertebrates: the immune response contrasting responses to micro- and macroparasites the costliness of host defense

Table 12.2 Estimated energetic costs (percentage increase in resting metabolic rate relative to controls) made by various vertebrate hosts when mounting an immune response to a range of 'challenges' that induce such a response. (After Lochmiller & Derenberg, 2000.)


Immune challenge

Cost (%)




Sepsis and injury


Typhoid vaccination


Laboratory rat

Interleukin-1 infusion




Laboratory mouse

Keyhole limpet hemocyanin injection





in the others. Evidence for this in vertebrates is reviewed by Lochmiller and Derenberg (2000), who illustrate, for example, the energetic price (in terms of an increase in resting metabolic rate) paid by a number of vertebrates when mounting an immune response (Table 12.2).

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