Innate versus adaptive immunity

We are all well acquainted with the phenomenon of infectious disease. Starting at birth, we live in a sea of microorganisms that colonize our skin, nose, throat, etc. It is, however, quite rare that these microbes make their way into our blood stream and tissues. This is because we are endowed with multiple defense mechanisms that promptly detect and kill the intruders. The microorganisms that manage to cross physical barriers such as the skin, will face the agents of innate immunity, the phagocytic cells. Phagocytosis, the engulfment followed by destruction of microbes, seems to be the most basic defense mechanism, present in all animals (1996). The cells that perform this function are called phagocytes. They are not only the major players of innate immunity, but also the connection between innate and acquired (adaptive) immunity. All vertebrates, starting with jawed fish, are endowed with adaptive immune systems. The defining feature of an adaptive immune system is its specific, inducible response to pathogens. The response is called specific when we can demonstrate that the body fluids of the infected animal contain cells or soluble molecules that react to the infective microorganism, but not to others, and inducible when we can demonstrate that the anti-microbial activity of the serum increases in response to the infection. Thus, the major distinction between innate and acquired immunity is that of scope. Phagocytic cells are general-purpose effector cells that can kill a wide variety of microbes, whereas lymphocytes, the agents of acquired immunity, are specific to a single microbe, and probably its very close relatives. The discriminative capacity of lymphocytes is useful in distinguishing microbial components from the components of the body. It is also what makes it possible for mutating microbes to evade the immune response.