将下列英文翻译。 Since antigens and antibodies are defined by their mutual interactions, they can each be used to quantify each other. Before we get down to details, it is worth posing the question ‘What does serum “antibody content” mean?’ If we have a solution of a monoclonal antibody, we can define its affinity and specificity with considerable confidence and, if pure and in its native conformation, we will know that the concentration of antibody is the same as that of the measurable immunoglobulin in ng/ml or whatever. When it comes to measuring the antibody content of an antiserum, the problem is of a different order because the immunoglobulin fraction is composed of an enormous array of molecules of varying abundance and affinity (figure 6.1a). Figure6.1 Distribution of affinity and abundance of IgG molecules in an individual serum.(a) Distribution of affinities of IgG molecules for a given antigen in the serum of a hypothetical individual. There is a great deal of low affinity antibody which would be incapable of binding to antigen effectively, and much lower amount of high affinity antibody whose skewed distribution is assumed to arise from exposure to infection.(b)Relationship of affinity distribution to positivity in tests for antigen binding. Rearranging the mass action equation, for all molecules of the same affinity K x and concentration of unbound antibody[Ab x ]: the amount of complex formed[AgAb] ∝ K x [Ab x ] for fixed [Ag].Starting with the lowest affinity molecules in the serum, we have charted the cumulative total of antibody bound for each antibody species up to and including the one being plotted. As might be expected, the very low affinity antibodies make no contribution to the tests. Serum 2 has more low affinity antibody and virtually no high affinity, but it can produce just enough complex to react in the sensitive agglutination test although, unlike serum 1, it forms insufficient to give a positive precipitin. Because of its relatively high ‘content’ of antibody, serum 1 can be diluted to a much greater extent than serum 2 and yet still give positive agglutination, i.e. it has a higher titer. The precipitin test is less sensitive, requiring more complex formation, and serum 1 cannot be diluted much before this test become negative, i.e. the precipitin titer will be far lower than the agglutination titer for the same serum. An average K a for the whole IgG can be obtained by analyzing the overall interaction with antigens as a mass action equation. But how can the antibody content of the IgG be defined in a meaningful way? The answer is of course that one would usually wish to describe antibody in practical functional terms: dose a serum protect against a given infectious dose of virus, does it promote effective phagocytosis of bacteria, does it permit complement-mediated bacteriolysis, and does it neutralize toxins, and so on? For such purposes, v ery low affinity molecules would be useless because they form such inadequate amounts of complex with the antigen. At the practical level in a diagnostic laboratory, the functional tests are labor intensive and therefore expensive, and a compromise is usually sought by using immunochemical assays which measure a composite of medium to high affinity antibodies and their abundance. The majority of such tests usually measure the total amount of antibody binding to a given amount of antigen; this could be a modest amount of high affinity antibody or much more antibody of lower affinity, or all combinations in between. Sera are compared for high or low ‘antibody content’ either by seeing how much antibody binds to antigen at a fixed serum dilution, or testing a series of serum dilutions to see at which level a standard amount of antibody just sufficient to give a positive result is bound. This is the so-called antibody titer. To take an example, as a serum might be diluted, say, 10000 times and still just give a positive agglutination test (cf figure 6.9). This titer of 1:10000 enables comparison to be made with another much ‘weaker’ serum which has a titer of only, say, 1:100. Note that the titer of a given serum will vary with the sensitivity of the test, since much smaller amounts of antibody are needed to bind to antigen for a highly sensitive test, such as agglutination, than for a test of low sensitivity, such as precipitation, which requires high concentrations of antibody-antigen product (figure 6.1b). To summarize: the ‘effective antibody contents’ of different sera can be compared by seeing how much antibody binds to the fixed amount of test antigen, or the titer can be determined, i.e. how far the serum can be diluted before the test becomes negative. This is a compromise between abundance and affinity and for practical purposes is used as an approximate indicator of biological effectiveness.