**Question:** Acellular pertussis vaccine contains-

A. Pertussis toxin
B. Filamentous hemagglutinin
C. Fimbriae
D. Lipid antigen

**Correct Answer:** A. Pertussis toxin

**Core Concept:** Acellular pertussis vaccines are a type of immunization that aim to protect against the bacterial disease pertussis (whooping cough). These vaccines contain parts of the pertussis bacteria, which are unable to cause disease when administered to humans.

**Why the Correct Answer is Right:** Acellular pertussis vaccines primarily contain pertussis toxin, which is a protein produced by the bacterium Bordetella pertussis. Pertussis toxin plays a crucial role in the pathogenesis of pertussis, and its components (S1, S2, and S3 subunits) are considered essential for inducing immunity against the disease. Vaccination with pertussis toxin stimulates the immune system to produce antibodies against the toxin, which in turn protects against the development of pertussis symptoms.

**Why Each Wrong Option is Incorrect:**

A. Filamentous hemagglutinin (FHA): This is a protein found in B. pertussis and contributes to the colonization of the respiratory tract. However, it is not a primary component of acellular pertussis vaccines.

B. Fimbriae: These are appendages on the surface of B. pertussis that enable adherence to host cells. While fimbriae are present in whole-cell pertussis vaccines, they are not the primary component of acellular pertussis vaccines.

C. Fimbriae: These are similar to option B, but repeated for clarity.

D. Lipid antigen: This refers to lipopolysaccharide (LPS) antigens produced by B. pertussis. Although LPS is present in whole-cell pertussis vaccines, it is not the primary component of acellular pertussis vaccines.

**Clinical Pearl:** Acellular pertussis vaccines have replaced whole-cell pertussis vaccines due to their reduced side effects and improved safety profile. These vaccines are composed of pertussis toxin and other antigens such as filamentous hemagglutinin and fimbriae, which are derived from Bordetella pertussis. The main advantage of acellular pertussis vaccines is their ability to induce immunity without triggering the severe side effects associated with whole-cell pertussis vaccines, making them a safer choice for mass immunization campaigns against pertussis.