What does feline leukemia virus primarily affect in cats?

Feline leukemia virus (FeLV) primarily affects the immune system in cats. This virus causes a severe compromise of the cat's immune system, making it vulnerable to various infections and diseases. By disrupting normal immune function, FeLV can lead to immunosuppression, which may result in other complications, including secondary infections, anemia, and potentially certain types of cancers, such as lymphosarcoma.

In contrast, while the respiratory, digestive, and central nervous systems can be affected in individual cases where secondary infections occur due to an impaired immune response, FeLV's primary mode of action is the suppression and alteration of the immune system itself. This foundational issue is what leads to the various clinical manifestations seen in cats infected with FeLV.