For microbes such as bacteria, fungi, viruses, and parasites, the inside of the human body represents a desirable place to grow and thrive (dark, warm, lots of nutrients). Fortunately, both the skin and our mucous membranes — tissues lining the nose, mouth, eyelids, digestive tract, and genital area, for example — usually work well to keep harmful invaders out of the body. When microbes make it past these protective coverings, though, your immune system’s organs, tissues, and cells are there to fight off the invaders.
To launch an immune response, your body has to be able to tell the difference between cells or substances that are “self” (part of you) versus “non-self” (not part of you and potentially harmful). All of your body’s cells carry specific proteins on their surfaces that help the immune system recognize them as “self.” That’s why the immune system usually doesn’t attack your body’s own tissues. (Autoimmune disorders occur when the immune system mistakenly attacks your own tissues, such as the thyroid gland, joints, connective tissue, or other organs.) “Non-self” materials have proteins and other substances on their surfaces that the body doesn’t recognize, called antigens. Antigens trigger the immune system to attack them and whatever they’re attached to, whether germs, viruses, bacteria, or something else. This response either destroys the foreign invaders or keeps them in check so they can’t harm the body.
On the following pages, you can learn more about the building blocks of the immune system and the immune response. We’ll also talk about why cancer cells are often able to get past the immune system and grow and thrive — even though it would seem like they are the ultimate “non-self” cells!