Which organism produces a factor that causes lymphocytosis?

Bordetella pertussis produces a factor known as pertussis toxin, which plays a key role in causing lymphocytosis. This organism is the causative agent of whooping cough (pertussis). The pertussis toxin acts by inhibiting the interaction of inhibitory G-proteins, leading to an increase in lymphocyte levels in the bloodstream. This mechanism results in the characteristic lymphocytosis observed in patients with whooping cough.

The increase in lymphocytes is partly due to the toxin's effect on the immune system, where it impairs lymphocyte trafficking and promotes their accumulation in the blood. This is a distinctive feature of Bordetella pertussis infection and helps differentiate it from other bacterial infections that may not typically cause such an immune response.