 Finally, provoking immune responses using unconjugated polysaccharides from the capsules of other bacteria, such as H. influenzae, has proven significantly more difficult.The pneumococcal conjugate vaccine consists of capsular polysaccharides covalently bound to the diphtheria toxoid CRM197, which is highly immunogenic but non-toxic.This combination provokes a significantly more robust immune response by recruiting CRM197-specific type 2 helper T cells, which allow for immunoglobulin type switching (to produce non-IgM immunoglobulin) and production of memory B cells.