Immunity is the term that describes the body's natural defenses against infection and disease. It is a combination of several layers of defense:
- Barriers and secretions. Our body is surrounded by an epithelial barrier that prevents microbes from gaining access to the blood and organs of the body, which are sterile when one is healthy. Skin and mucosal membranes at every opening to the body (mouth, nose, ears, pores in skin, and many others) provide barriers for this purpose. These barriers also feature protective secretions, whether antibody-containing saliva or oils secreted through the skin or mucous produced by the respiratory and digestive systems. The secretions contain antimicrobial chemicals and molecules that make colonization by invaders a difficult task. They also trap microbes and irritants that are flushed from the body (cough, sneeze, vomiting, diarrhea) to keep tissues as microbe-free as possible.
- Innate ("nonspecific") immune activities. When those barriers are breached, the body reacts to the damage caused by releasing signal chemicals that start natural responses to fight off infection.
- The site of injury is often modified by the inflammatory response, which prevents spread of any invaders and mobilizes white blood cells to locate and destroy any microbes gaining access. The inflammatory response is characterized by redness at the site of injury (the chemicals dilate blood vessels to allow white cells to come to the injury, which in turn allows more red cells to the area and thus the reddish color of injured tissues), inflammation (there is a localized increase in temperature, like a local fever, that makes conditions unfavorable for microbial growth but more favorable for white cell activity, pain (chemicals released touch off free nerve endings to alert the brain to the injury), and swelling (the dilated capillaries allow fluids to escape and thus swelling in adjacent tissues; this produces osmotic pressure to prevent the microbes from moving "upstream" into the rest of the body).
- Additionally, there are substances within the body that are capable of finding and disposing of the invaders when these conditions are established. And, there is the process of phagocytosis to engulf and digest any invaders to prevent growth and spread. All of these activities take place regardless of the nature of the injury to the barrier - whether caused by bacteria, virus, pollen, splinter, or glass shard.
- Adaptive ("specific") immunity. In cases where these measures prove insufficient to prevent infection and disease, the body takes things very personally! It responds with a very specific full-out assault that involves T-cells and B-cells and antibodies. The activities of innate immunity and phagocytosis result in the invader's antigens being presented to immunocompetent T-cells and B-cells in a real-life adaptation of the glass slipper and Cinderella. The antigen is shopped around to immunocompetent T-cells and B-cells in an attempt to find one that has a cell surface receptor with a shape and size and charge that allows perfect binding with the antigen. The matching T- and B-cells then go through rapid proliferation and differentiation (growth in numbers, variation in types of cells produced) and mount a specific immune response solely toward that recognized antigen. Anything with the antigen on it is either bound to antibodies and thus tagged for destruction or encounters a specific cytotoxic T-cell (killer T-cell) and is destroyed outright. So adaptive immunity is not like the innate immunity in its specificity. Adaptive will only attack cells sporting the recognized antigen, whereas innate provides a general response to anything foreign to the body.
Click hereto visit the Howard Hughes Medical Institute website and view an overview (slides and videos) of the blood cells and their roles in innate and adaptive immunity.
With this background in mind, there are two VUMIEtm 2012 activities that will provide further insight into these two different forms of immunity, and a Scavenger Hunt to allow you to apply the knowledge gained to understanding some aspects of public health.