Which body system is affected if an individual has trouble with their spleen?

The immune system is the correct choice in this context because the spleen plays a crucial role in the body's defense mechanisms. It is an integral component of the immune system, responsible for filtering blood, removing old or damaged red blood cells, and serving as a site for the activation and proliferation of immune cells, particularly lymphocytes. When the spleen is not functioning properly, the body may struggle to effectively respond to infections and other immune challenges, leading to increased susceptibility to illness. Therefore, issues with the spleen directly impact immune function, making it vital to understand this connection in the context of health and disease.