The immune system normally provides continuous surveillance of body processes and acts to remedy imbalances or abnormalities. It protects the body from the effects of invasion by microorganisms (e.g. germs) and governs the removal of damaged cells from body tissue. It also discovers and disposes of abnormal cells that continually arise within the body. Immune system failure can lead to vulnerability to infection, and can also allow tumor cells to grow unchecked. Immune system overactivity can cause allergic symptoms.