Lymphopenia

In FIP, a mutated feline coronavirus infects macrophages, which then spread the virus throughout the body.

This triggers a strong immune response with widespread inflammation and vasculitis (inflammation of blood vessels), the hallmark lesion of FIP.

The persistent inflammatory state and cytokine release cause lymphocyte depletion and redistribution, resulting in lymphopenia, one of the most common CBC abnormalities seen in affected cats.

Therefore, lymphopenia reflects the severe systemic inflammation and immune dysregulation.