Answer:
His immune system cannot fight against invading pathogens.
Step-by-step explanation:
Weaken immune system represents a state of immunodeficiency, in which immune system of an individual fails to fight properly with infections and cancer.
This is due to inability of body to make functional immune cells, such as neutrophils, T cells, B cells, and macrophages or decreased production of these cells and other components of immune system that provide protection against invading pathogens.