Some people get sick more often than others, even though our body is attacked everyday by several infectious agents, we do not succumb to every agent. Immunity of our bodies varies from individual to individua. Immunity is the overall ability of a host to fight the disease-causing organisms. Immunity is of two types: innate and acquired. Innate immunity is something we all possess since birth.
Four different types of barriers are provided by innate immunity: physical barriers, physiological barriers, cellular barriers and cytokine barriers.
The skin on our body is the main physical barrier to the entry of microbes. The mucous coatings of the epithelial lining of the respiratory, gastrointestinal and urogenital tracts inside the body are also physical barriers to microbes as they trap them.
Saliva in the mouth, hydrochloric acid inside the stomach as well as tears in the eyes act as physiological barriers to microbial invasion.
Our body also has certain types of leukocytes such as monocytes, natural killer lymphocytes and polymorpho-nuclear leukocytes in the blood. Together, they constitute a cellular barrier along with macrophages in the tissues. They can also destroy microbes.
Some virus-infected cells release proteins called interferons that prevent the spread of infection in the body, thereby acting as cytokine barriers in the body. On the other hand, acquired immunity is pathogen-specific. It depends on the body’s memory.When body encounters a bacterium for the first time, it produces a low-intensity response called a primary immune response and produces memory cells. However, when the same bacterium attacks again, the memory cells help to elicit a secondary immune response of high intensity. This response is also known as an anamnestic response. Both primary and secondary immune responses are made possible by two special lymphocytes in our blood: B-lymphocytes and T-lymphocytes respectively. B-lymphocytes produce proteins to fight pathogens whereas T-lymphocytes help B-cells to perform this task.
The protective proteins produced by B-cells are called antibodies. Each antibody molecule has four peptide chains, two small and two longer ones. The two small chains are called light chains while the two longer ones are called heavy chains.
Acquired immunity is primarily of two types: a humoral immune response and cell-mediated immunity (CMI). It is the humoral immune response that is carried out by antibodies. The word ‘humoral’ refers to blood or any other body fluid. The cell-mediated immune response can be best understood with the example of an organ transplant. When organs like the eye, heart, kidney or liver fail, organ transplant is the only remedy.
The ability of the body to differentiate between ‘self’ and ‘non-self’ while rejecting grafts is the cell-mediated immune response. It is an immune response that doesn’t involve antibodies but it involves the activation of macrophages, natural killer cells (NK), antigen-specific cytotoxic T-lymphocytes and the release of various cytokines that fight antigens. Patients who have had organ transplants have to take immune-suppressants all their lives to inhibit their bodies’ immune response against the transplant. Then again, based on whether the antibodies are produced inside or outside the body, immunity can be distinguished as active immunity or passive immunity.