The genetic-reductionist approach suggests that there is only a self signified by a genetic marker. This theoretical position implies that the nonself is not an actual entity but a synonym for a genetic foreigner. The opposite perspective was presented by Burnet in his clonal selection theory (CST). He suggested that lymphocytes with reactivity to host constituents are destroyed during development, and only those lymphocytes that are nonreactive are left to engage the antigens of the outside world. The foreign object is destroyed by the immune cells and their products, whereas the normal constituents of the organism are ignored. That is, the immune system recognizes only the nonself - and the self is an empty term. Burnet's CST explains from a very simple evolutionary perspective why we tolerate ourselves. We tolerate ourselves because those who were unable to tolerate themselves (i.e. differentiate between self and nonself) did not survive.
There are major difficulties with Burnet's conception of the immune self. One is the fact that self-recognition is clearly evident in the immune system (Cohen, 1994). I will point out these difficulties in the following sections, but for the present phase of our analysis, I would like to note some similarities between Burnet's conception of the self and Saussure's conception of the sign.
Burnet's concept of the self is purely differential and negative. The self exists only as the background for identifying the foreign object, the nonself (Cohen, 1994). In a certain sense, this position is similar to the one presented by Ferdinand de Saussure in his classical text Course in General Linguistics. According to Saussure (1972, p. 118), "In the language itself, there are only differences. Even more important than that is the fact that, although in general a difference presupposes positive terms between which the difference holds, in language there are only differences and no positive terms."
What does he mean when he says that in language there are "only differences"? For Saussure, language as an abstract system (la langue) is "a system of distinct signs corresponding to distinct ideas" (1972, p. 26). That is, in itself a sign means nothing. It exists solely by being differentiated. According to this interpretation, the sign "cat" has no intrinsic meaning. The "catness" of the cat is not embedded either in the way "cat" is pronounced or in concept of a cat. The same is true of our self.
During our lifetime, our self changes significantly: cells die and are replaced by new ones, our mental content changes during our development, and so on. There is nothing intrinsic to our self that can define us as the same person over the years. According to this line of reasoning, our identity is primarily and negatively established by differentiation from others. To use an analogy from mathematics, a pair of points consists of units that are indistinguishable in isolation. Each unit in the pair is distinct only in that its position is different from the other.
Saussure's statement is applied to the sign as an isolated unit that is "purely differential and negative" (1972, p. 118) as a phonetic or a conceptual unit. Bear in mind that for Saussure the meaning of a word is the "counterpart of a sound pattern" (p. 112). In this sense the meaning of the sign "cat" is the corresponding concept of a cat. Saussure suggests that meaning should be distinguished from value, which is important for understanding the abstract nature of any system of signs.
A value involves "(1) something dissimilar which can be exchanged for the item whose value is under consideration, and (2) similar things which can be compared with the item whose value is under consideration" (Saussure, 1972, p. 113). For example, money is an abstract system of signs/values. In this system, as in the linguistic system, a dollar bill has no intrinsic meaning. The meaning of a dollar bill can be determined only in a closed system of values. To determine the value of a dollar bill, we should know that it can be exchanged for something else (e.g. ice cream) and that its value can be compared to another value within the same system of currency (e.g. it can be exchanged for euros). The linguistic system is a system of pure values whose function is to combine the two orders of difference - phonic and conceptual - in the making of signs.
Turning to immunology, the similarities are clear: the immune self has no intrinsic meaning. The immune self is only negatively established through the existence of the other, nonself. However, at the point where Burnet stops his analysis, Saussure presents a system-oriented approach, moving away from the isolated sign in language as an abstract system and pointing to the social semiotic dynamics that flesh out this abstract system of values in practice. Surprisingly, Saussure's theory of language as a social network of signs is highly relevant for understanding self and nonself discrimination.
As suggested by another semiotician without any reference to immunology (Thibault, 2005, p. 4), "Meaning is an embodied relation between self and nonself on the basis of the individual's entraining into the higher-order and transindividual structures and relations of langue." In simple words, this means that only by going beyond the individual level of analysis and entering the semiotic network can the relation between self and nonself be clarified. As will be shown in the Section 5, this statement has clear relevance for studying the immune self.
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