The human immune system is made up of chemicals, cells and organs that cohesively fight against infection. The white blood cells, spleen, thymus, bone marrow, complement system, antibodies and lymphatic system actively participate to fight against toxic foreign particles in the body.
Not only does the immune system fight, but it builds a memory of every foreign body that has ever been defeated. This memory helps in generating a quick response if the infection enters the body again. This inbuilt response against infection is called innate immunity.
However, some infections such as cold and flu can occur due to different viruses. In such a scenario, the immune system may not have a memory against all of them. Due to this, people often feel sick due to cold and flu.
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