Both Nonimmune and Immune Mechanisms Defend Against Invading Antigens
The body is confronted with literally billions of antigens. Consequently, a unique challenge presented to the immune system is to respond effectively only to foreign antigens while refraining from response to “self” antigens.
The induction of immune responses requires energy and protein, and antigens require extensive cellular division (and hence utilization of protein reserves), the body cannot mount immune responses to each of the innumerable antigens it encounters constantly. Instead, the body is well equipped to handle antigens effectively before resorting to a specific immune response.Initially, most antigens are effectively handled by nonspecific defense mechanisms, such as impervious and formidable physical barriers (e.g., skin and other body surfaces) and antimicrobial body fluids (e.g., lysozymes in tears, saliva, gastric juices). These are considered the first line of defense and are discussed next. Should the antigen survive this “body armor,” phagocytic cells (e.g., neutrophils, macrophages- monocytes) and natural killer (NK, NK-T) cells can effectively eliminate the invading antigens. These cells ingest and destroy a wide range of antigens and thus are non-antigen specific. These cellular defenses constitute a second line of the body’s defense. The body’s initial defense (physical, chemical, and phagocytic, antigen-presenting cells, and natural killer cells) constitute the innate immune system. The antigen-presenting cells closely interact with specific T and B cells to induce a specific immune response. Thus, specific immune responses by the “adaptive” immune system tend to be the last line of the body’s defense (see Chapter 55). Collectively, both non- immune and immune mechanisms effectively counter invading microbes.