Our immune system defeats the foreign pathogens by two independent and distinct biological processes. Pathogens can be viruses, bacteria, fungi, nematodes and the like that can create disease or discomfort in to the body. Immunity illustrates biological defenses through which living entities including human fight infection or diseases or discomfort. Immunity also includes biological processes to prevent biological invasion.
One immunological defense system is known as the innate immune system or innate immunity. Human is born with innate immunity. Innate immunity is the natural way of providing a defense or resistance to invading microbes through physical or chemical or cellular methods. The physical barriers include the skin and the pathways that lead towards the inside of the body; the mucous substances such as the saliva, the tears and the like provide the physical barriers. The chemical barriers include the antimicrobial agents, the chemical signaling substances such as cytokines etc., which are secreted by phagocytic cells that encounter the microbes; such encountering can result in inflammation and fever. All these symptoms and secretions are defensive against the invading microbes. Once the invading microbes are inside, the body triggers immediate encounter to the cause of infection without being specific to the disease-causing microbes. The immune cells within the human body, which include the natural killer cells, mast cells, eosinophils, basophils, and the phagocytic cells including macrophages, neutrophils and dendritic cells mount the offence for the defense of the body. The attack is primitive in the sense that these immune cells and their cell-surface receptors straight –way recognize certain molecular pattern present on the surface of the pathogens, which are uniquely linked or associated with several kinds of disease producing agents. Recognition of the molecular patterns, which are associated with diseases (in animals) have been developed through evolution.
As the cells of the innate immune system recognize only general molecular pattern, which are residing on the surfaces of the pathogens and which also include toll-like receptors (TLRs), the attack of innate immune system is non-specific. Moreover, many pathogens over the years have learnt and developed ways to evade the recognition by changing and mutating its surface molecular patterns. Therefore, the innate immune system cannot recognize them and thus have limited capacity in resisting some of the specific infections; in such situations, the microbes acquire the capacity to evade recognition and may infect the recipients.
The other immune system, known as the adaptive immune system is highly specific in recognizing the pathogens; here the recognition system is extremely precise. This system can virtually recognize any pathogen that the body might encounter; this system also provides immunological memory of infection. In this system, two types of responses are observed. In one type, specific B-cells are activated and multiplied, which produce protein antibodies (also known as sensitizes immunoglobulins) that can bind on to the surface of the invading microbes or their parts (against which recognition has been established by the immune system). Such antibodies can also be produced naturally in human after an infection or a child from the mother can acquire them. These can also be produced in-situ in human through vaccination, when an individual is artificially injected with a safe antigen (vaccine) or an attenuated (non-disease producing) microbe (vaccine). In certain cases, disease-sensitized immunoglobulins prepared elsewhere, can be injected in to individuals to impart immunity against a disease.
In the other type of adaptive immune system, highly lethal killer T-cells are created within individuals; these T-cells (called as the activated CD-8 positive T-cells) can recognize the invading microbes and therefore, can attack them for destruction.
The adaptive system of immunity relies upon the recognition of a pathogen through the innate immune systems to initiate its own response to a pathogen. Therefore, the adaptive system is slow compared to the innate immune system.
The two systems, namely the innate and the adaptive immune systems together provide a comprehensive defense response to infections.