A deficiency in which complement component leads to increased susceptibility to encapsulated organisms like pneumococcus?

The complement system is an essential part of the immune response, and different components play unique roles in fighting infections. A deficiency in complement component C3 is particularly significant because C3 is a central component in the complement cascade, crucial for opsonization and the clearance of pathogens.

Encapsulated organisms like pneumococcus are known to evade the immune system through their protective polysaccharide capsules. The role of C3 in opsonization means that when C3 is deficient, the ability of the immune system to mark these encapsulated organisms for destruction by phagocytes is impaired. This leads to an increased susceptibility to infections by such organisms, as the immune system lacks the necessary signals to efficiently recognize and eliminate them.

While other complement components like C1, C2, and C4 are involved in other aspects of the immune response, such as initiating the classical pathway, they do not have the same level of direct impact on the opsonization process, particularly with encapsulated bacteria. Therefore, a deficiency in C3 notably contributes to an increased risk of infections by encapsulated pathogens like pneumococcus.