Describe the function of the immune system.

The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful invaders such as bacteria, viruses, and disease-causing microorganisms, also known as pathogens.

Here's a step-by-step description of how it functions:

1. Recognition: The immune system identifies foreign substances or pathogens in the body. This is done by the white blood cells, which are the first line of defense. They circulate in the bloodstream and lymphatic system.

2. Activation: Once a pathogen is recognized, the immune system activates the appropriate immune response. This could be the innate immune response, which is non-specific and reacts the same way to all pathogens, or the adaptive immune response, which is specific and targets particular pathogens.

3. Response: The immune system responds to the pathogen by producing proteins called antibodies. These antibodies are designed to target the specific pathogen that has been identified. They bind to the pathogen, marking it for destruction.

4. Elimination: The immune system then eliminates the pathogen from the body. This is done by cells called phagocytes, which engulf and destroy the pathogen.

5. Memory: After the pathogen has been eliminated, the immune system 'remembers' it. This means that if the same pathogen tries to invade the body again, the immune system can respond more quickly and effectively.

In summary, the function of the immune system is to protect the body from harmful invaders. It does this by recognizing and responding to pathogens, eliminating them from the body, and remembering them so it can respond more effectively in the future.