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Time in Weeks Figure 5.7 • Diagram of the time course of the anamnestic (memory) response. After the first immunization (week 1), the titer of Ab increases slowly to a low level and wanes. A challenge with the same antigen (shown in the diagram at week 4) elicits rapid development of a high titer of Ab in the blood.

targets. Examples of toxins are tetanospasmin (tetanus toxin; Clostridium tetani) and diphtheria toxin. Both react with specific receptors in the inhibitory interneurons of the nervous system, causing spastic paralysis or flaccid paralysis, respectively. When an Ab blocks the toxin's receptor-binding region, it can no longer bind to the neural receptors and is rendered harmless. The toxin-Ab complex is soluble and requires no further processing. The complex can then

Figure 5.8 • Combining ratios of Ab and antigen. Because the Ab is bivalent, there exist a set of conditions under which optimal proportions yield a stable lattice structure, neutralizing the antigen. If antigen or Ab is in excess, the lattice will not form.

be eliminated by the kidneys. Bacteria are immobilized by neutralization.

Precipitation

When a soluble antigen reacts with an Ab, it may form an insoluble particulate precipitate. Such a complex cannot remain in the bloodstream in its insoluble state. These species must be removed by the spleen or through the reticuloen-dothelial system by phagocytosis.

Agglutination

Bacterial cells may be aggregated by binding to antibodies that mask negative ionic surface charges and cross-link cellular structures (Fig. 5.8). The bacteria are thus immediately immobilized. This limits their ability to maintain an infection, but it forms a particulate matrix. This type of complex must also undergo elimination through the reticuloendothe-lial system.

Bacteriolysis

Bacteriolysis is a complement-mediated reaction. The last five proteins in the cascade self-assemble to produce a membrane attack complex that disrupts the cell membranes of bacteria, acting like bacitracin or amphotericin B. The cell membranes lose integrity, cell contents leak out, membrane transport systems fail, and the cell dies. This type of reaction yields products that require no special treatment.

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