The immune system is a large network of organs, white blood cells and proteins that work together to protect you from germs that could make you sick. It recognizes cell changes that indicate an infection, then attacks and kills invading microbes, limits the damage they do, and remembers what these invaders look like so it can quickly respond if the germs come back.
Infections that you’ve had before cause your body to develop antibodies against those specific invaders. This is known as natural or adaptive immunity. You also get some adaptive immunity from vaccines, which are made by fusing antibodies with other materials that help trigger the right response.
Immune system cells have receptors on their surfaces that can bind to antigens, which are proteins from the germs or other foreign substances that invade your body. When these cells bind to an antigen, they stimulate other cells to respond. This process is coordinated by the release of molecules called cytokines that promote cell-to-cell communication and activate immune cells.
After the innate immune response, B cells and T cells develop into memory cells that remember the particular pathogen and can respond quickly to it in the future. The memory cells can produce antibodies that attack the pathogen and mark it for destruction by phagocytes.
Mast cells and basophils also act in the inflammatory response to help kill the pathogen. They release chemicals, including histamines, that increase blood flow to the site of infection and permeability in surrounding tissues. They also release preformed mediators, such as leukotrienes and prostaglandins, that help with the allergic response by causing vasodilation and bronchoconstriction.