The immune system is complex with many aspects. It is spread through the body but less active in some areas (inside joints, the brain and spinal cord) and concentrated in others (the intestines, neck, lungs and underarms). There are 2 main parts to immunity - the "Innate" and the "Humoral" systems. The innate immunity includes the white blood cells (phagocytes) you are probably familiar with, which can go to a site of invasion within a few hours and engulf and destroy invaders or toxins. It also includes some physical barriers like mucus and some chemical cytokines. The "humoral" or "adaptive" immunity is the more specific system that takes 4-5 days to react, and includes T-cells and B cells that produce specific antigens to a bacteria or sometimes a body tissue. This is the type of immunity that comes from getting an immunization or exposure to a virus or bacteria and usually lasts for years. It is also the type of immune reaction involved in allergies.
The innate immune response is called to the site of possible invasion by "chemotaxis" or chemical signals. It uses controlled inflammation as one of the tools to destroy invaders. They include some "antigen presenting cells" that prime the adaptive immune response.
Next time we will talk more about how this process can go wrong.