Immune system

Your body has a built-in way to stay safe from many dangers. This is called the immune system. It is a huge network of biological systems. Its main job is to protect you from diseases. It looks for pathogens, which are tiny things that can cause sickness. These include viruses, bacteria, and parasites. It can even find cancer cells or foreign objects like wood splinters. The system is very smart. It can tell the difference between a germ and your own healthy tissue.

This system works using layered defenses to keep you healthy. First, physical barriers act like walls to stop germs. Your skin is a great example of a mechanical barrier. Other barriers include the waxy coating on leaves or insect shells. Even your tears and saliva help by flushing germs away. If a germ gets past these walls, the innate immune system jumps in. This system provides a fast response to many different germs. It uses special cells called phagocytes to find and swallow invaders. Once a germ is inside a cell, it is trapped in a tiny bubble. Then, enzymes work to kill the germ.

Vertebrates, which are animals with backbones, have a second layer of protection. This is the adaptive immune system. It is activated after the innate system does its work. This system is special because it learns about each specific germ. It recognizes molecules called antigens. Antigens are substances that trigger an immune response. The adaptive system creates an immunological memory. This means it remembers a germ after the fight is over. If that same germ tries to attack again, the body reacts much faster. This memory is the main reason why vaccinations work so well.

Many different types of cells help the immune system work. Neutrophils are the most common phagocytes in your blood. They often arrive first at a site of infection. Macrophages are also very important cells. They act like scavengers to clean up old cells and debris. Dendritic cells are shaped like they have many tiny spines. They act as a bridge between the innate and adaptive systems. There are also natural killer cells. These cells do not attack germs directly. Instead, they destroy your own cells if they become infected by a virus.

Sometimes, the immune system does not work the way it should. If it is too active, it might attack your own healthy body. This is called an autoimmune disease. Common examples include rheumatoid arthritis or type 1 diabetes. Other times, the system might be less active than normal. This is called immunodeficiency. It can be caused by genetics or conditions like HIV/AIDS. When this happens, it is much harder for the body to fight infections. Scientists who study these systems are called immunologists. They work hard to understand how to keep our defenses strong.