B Cells Are Activated by CD4 T_H2 Cells Only If

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The Answer is A. Many bacteria are surrounded by a polysaccharide capsule. In some cases, antibodies against the capsular polysaccharides give protective immunity against the pathogen. Antibodies produced against polysaccharide antigens are generally restricted to the IgM isotype, because the help needed to switch isotypes to IgG is provided by T cells, which recognize only peptide antigens. Adult humans make effective immune responses to polysaccharides alone and thus can be protected by subunit vaccines made from the capsular polysaccharides of encapsulated bacteria. Their antibody responses are polysaccharide-specific, T-cell independent, and involve antibodies of the IgM isotype. In contrast, children do not make effective immune responses to polysaccharides alone and thus cannot be immunized with such vaccines.
However, if the polysaccharide is conjugated to a protein, peptides from the protein part of the molecule can activate specific TH2 cells. B cells specific for polysaccharide will bind and internalize the whole antigen via their antigen receptors, process it, and then present peptides from the protein part on their surface. T cells specific for these peptides will interact with the B cell, delivering the necessary cytokines (such as IL-4) and the CD40–CD40-ligand signal required for isotype switching. The B cell will then produce IgG anti-polysaccharide antibodies. This type of vaccine can be used to immunize children so as to induce protective anti-polysaccharide antigens.
B. A vaccine of this type has been produced against Haemophilus influenzae B (HiBC), which can cause pneumonia and meningitis. The conjugate vaccine is composed of a capsular polysaccharide of H. influenzae conjugated to tetanus or diphtheria toxoid (a protein). The antibody response is polysaccharide-specific, T-cell dependent, and comprises IgG that protects children from the meningitis caused by this microorganism.

B Cells Are Activated by CD4 TH2 Cells Only If

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B Cells Are Activated by CD4 T_H2 Cells Only If