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Chapter 8 : Bacterial Resistance to Antibody-Dependent Defenses
Bacterial disease results from a dynamic interaction between pathogen and host. Antibody protects against bacterial infection in numerous ways. Agglutination (or clumping) and immobilization of motile organisms are among the conceptually simplest defense mechanisms. Gene cloning and sequencing revealed that leukotoxin (LKT) is part of a family of RTX toxins. Gene cloning and sequencing revealed that LKT is part of a family of RTX toxins. Other members of the family contribute to evasion of antibody defenses as well. Several gram-negative bacteria block the antibody activation of the C cascade, resulting in escape from C-mediated killing. Bacteria lacking a protective coat such as a capsule may avoid antibody-mediated defenses by shedding antigen into the external milieu. Antigenic variation is a mechanism of evading a vigorous antibody response. Some years ago, it was noted that the lipooligosaccharide (LOS) of Haemophilus somnus isolated from calf lungs in a chronic pneumonia study had LOS with different migration patterns in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) than the LOS of the infecting strain. Microbes divert immune responses away from productive, protective immunity in several ways. Motifs discovered in the sequence analysis provided a basis for functional studies of the protein to aid in understanding its relevance to evasion of antibody-mediated defense. These should help to explain the persistence of H. somnus infection in calf lungs for at least 10 weeks in the presence of specific antibody.