Immunity to Diseases All forms of living creatures in the earth are vulnerable for predation by others. Most invertebrate and vertebrate species are highly susceptible to diseases caused by pathogenic microbial organisms. The invertebrate and vertebrate species in turn have developed a strong army of killing machineries to contain the attacking parasites, called immune system that consists of many policemen cells in a living body. The phagocytes (neutrophils and macrophages) are the frontline soldiers that protect body systems non-specifically against most of the foreign invaders, while lymphocytes provide specific immunity with high precision in recognition and destruction of the foreign targets. The interactions between phagocytes and lymphocytes play a major role in controlling the infectious diseases caused primarily by microbial pathogens (Figure). Establishing a protective immunity against the pathogenic microbial organism requires the greater skill by immune cells to recognize and attack the foreign invaders. Although immune cells (phagocytes and lymphocytes) are capable of killing pathogenic organisms in precision, the microbial pathogens in turn posses various evasive strategies that helps them to survive inside the host cells for a prolonged period. This is a critical phenomenon for those pathogens induce chronic diseases (eg. tuberculosis), while the outcome of acute diseases (eg. pneumonia by influenza or SARS virus) caused by several pathogens are a win-win situation for pathogens as well as hosts that resulting in a transient infectious state to sudden death. Thus, the outcome of a disease caused by microbial pathogen is, in general, a complex process ranging from a short course of ill-state to chronic prolonged disease condition, sudden death or progressive wasting of body system and subsequently moribund state. |