Our immune system implements various strategies to protect us from pathogens. In general, we can summarize the various lines of defense implemented by the immune system into four categories: physical barriers, general recognition, specific recognition, and memory of the infection. Below I outline a brief description of each of these strategies. Follow up articles will dive deeper into each of these topics.
Physical barriers. The first line of defense from infection are the physical and chemical barriers presented by the body. These barriers can physically prohibit infection by blocking entry into the body (e.g., epithelial cells) or slow or kill certain microbes (e.g., the antimicrobial peptides found at mucosal surfaces). These barriers are, obviously, not always effective.
General recognition (innate immunity). The innate immune system’s role is to recognize infecting agents in a general sense. It is not specific about what it looks for, but rather it searches for those things that have made their way into the body. This tends to be the first line of defense from a pathogen that has subverted the physical barriers of the body. Because the mechanisms it invokes tend to be broad-based, it is not necessarily the most efficient means of clearing a pathogen. But, it tends to be very good at identifying that an infection is occurring, localizing or containing the infection, and recruiting the adaptive immune system to clear the infection.
Specific recognition (adaptive immunity). By contrast, the adaptive immune system provides an efficient means of clearing an infection, but isn’t always good at initially recognizing that an infection is taking place. Here, the recognition of pathogen specific antibodies induces a cascade of immune cells that target the pathogen and expedite its clearance from the body. The innate immune system usually triggers the activation of the adaptive immune system.
Memory of the infection (immunological memory). Fortunately for us, once our immune system has cleared an infection, it develops a memory of the pathogen to expedite its detection during a subsequent infection. This strategy makes it very difficult for that pathogen to successfully reinfect our bodies (though we have to watch out for evolution). When scientists develop vaccines, they are effectively hijacking this process.