How can the immune memory lead to the efficacy of vaccines and also produce allergies?

Vaccines are controlled inoculations of fragments of infectious agents or of inactive infectious agents to persuade the primary immune response, the formation of exact memory B lymphocytes against the antigen. Thus the organism produces immunoglobulins and becomes prepared to destroy antigens when exposed to new infections by those agents.

In allergies the humoral immune system is sensitized (makes antibodies and exact memory B lymphocytes) against some common environmental substances wrongly standard as antigens. For example, pollen-derived substances, dust particles, compounds present in foods or in medicines, etc. might be recognized as antigens triggering the primary response and making an immune memory against them that then become causes of allergy. The more the individual is exposed to those substances the more intense is the immune reaction.