An 80-year-old woman, a retirement home resident, has multiple bouts of pneumonia caused by Streptococcus pneumoniae. In an attempt to prevent such infections, polyvalent vaccines directed at multiple serotypes of the organism have been administered but have not elicited long-acting immunity. Which of the following is the probable explanation for this phenomenon?

A. Memory T lymphocytes respond poorly to polysaccharide antigens.

B. S. pneumoniae evades host immune response response by forming capsular coatings composed of host proteins and recognized as "self" antigens.

C. The bacterial capsule binds C3b, facilitating activation of the alternative complement pathway, inducing complement-mediated lysis, and preventing immunization.

D. The capsular polysaccharides of S. pneumoniae have limited hapten potential

Correct Answer: Memory T lymphocytes respond poorly to polysaccharide antigens.

Description: Antibody responses to the more than 80 differing carbohydrate capsular antigens of the various strains of S. pneumoniae are generally T-cell-independent, and antibody formation is entirely B-cell-mediated. Because of this, memory cells are not formed, and long-lasting immunity is not achieved.

Category: Pathology

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