**5. Immunological aspects**

After natural infection, anti-PT antibodies (the only *B. pertussis* specific antigen) are found in 80–85% of patients [2]. Antibodies to different *B. pertussis* antigens are believed to play a key role in protecting from the disease (as they neutralize bacterial toxins, inhibit the bond between the bacterium and the respiratory tract cells, and allow the capture and destruction of the bacterium by macrophages and neutrophils). Nevertheless, any specific antibody level, against a single antigen or a combination of antigens, which can be related to clinical protection, is currently unknown [21].

Immunity, whether natural or acquired by vaccination, is not long-lasting and tends to decline in a 4–12 years time range. This data is confirmed by the occurrence of epidemics especially in adolescents and adults, even in geographical areas where vaccine coverage is high. Reinfections may occur in adolescents and adults and have been reported in children as well. It is also well known that cell-mediated immunity plays a key role in protecting against infection; the development of this response can be very important in the clearance of the microorganism and in the subsequent protection [22, 23].

Although there is a placental transmission of maternal antibodies, most newborns do not appear to be protected against the disease during the first months of life, probably due to the low and inadequate levels of antibody transferred, unless the mother has been recently vaccinated. Several studies on maternal immunization have evaluated its validity, demonstrating an effective antibody-mediated protection of infants [24].
