Final answer:
Patients with hyposplenism or post-splenectomy are at risk for severe infections due to the absence of B lymphocytes from the spleen, which are essential for producing antibodies against pathogens like Streptococcus pneumoniae.
Step-by-step explanation:
Patients with hyposplenism or those who have undergone splenectomy are at risk for overwhelming pneumococcal infection because they lack a certain splenic element that is crucial for the production of protective antibody against pneumococcal cell wall and other carbohydrate antigens. This cell population consists of B lymphocytes, which are a central component of the adaptive immune system. B lymphocytes are responsible for the production of antibodies, and the spleen is a key organ where these cells can encounter pathogens, become activated, and differentiate into plasma cells that secrete antibodies. After a splenectomy or in cases of hyposplenism, the absence of this function leads to an increased susceptibility to infections, particularly by encapsulated bacteria such as Streptococcus pneumoniae.