Aperson who has a weakened immune system may not be able to fight against infection because
a) pathogens may enter the body and cause illness.
b) phagocytes many enter the body and cause illness.
c) normal cells crowd out unhealthy cells.
d) of an increase in the production of antibodies.

A) pathogens may enter the body and cause illness.  

Explanation:

The human immune system (or immune or immune system) consists of a network of cells, tissues and organs that act in defense of the organism against the attack of external invaders. These invaders can be microorganisms (bacteria, fungi, protozoa or viruses) or harmful agents such as toxic substances (eg venom from venomous animals).  Substances foreign to the body are generically called antigen. Antigens are fought by substances produced by the immune system, of a protein nature, called antibodies, which react specifically with antigens.

When a person has a weakened immune system, the cells in the immune system cannot fight off invaders effectively, the body may react with disease, infection or allergy because pathogens can enter the body and cause disease.

A

Explanation: