This fragile first-shaped sack in your body’s infection fighter, filtering bacteria, viruses and other nasty invaders from your blood. Your spleen's main function is to act as a filter for your blood. It recognizes and removes old, malformed, or damaged red blood cells. When blood flows into your spleen, your spleen performs "quality control"; your red blood cells must pass through a maze of narrow passages. Healthy blood cells simply pass through the spleen and continue to circulate throughout your bloodstream. Blood cells that can't pass the test will be broken down in your spleen by macrophages. Macrophages are large white blood cells that specialize in destroying these unhealthy red blood cells.
The blood vessels in human spleens are able to get wider or narrower, depending on your body's needs. When vessels are expanded, your spleen can actually hold up to a cup of reserve blood. If for any reason you need some extra blood – for example, if trauma causes you to lose blood – your spleen can respond by releasing that reserve blood back into your system.
Your spleen also plays an important part in your immune system, which helps your body fight infection. Just as it detects faulty red blood cells, your spleen can pick out any unwelcome micro-organisms (like bacteria or viruses) in your blood.
Picture Credit : Google