Our immune system gives us protection from infectious diseases. Immunity is a state of protection from harmful bacteria, viruses, parasites etc. The immune system is spread throughout the body and involves many types of cells, organs, proteins, and tissues. It helps to distinguish our own tissue from foreign tissues. Immune system recognizes dead and unnecessary cells and cleared them out. If our immune system encounters a pathogen ( a bacterium, virus, or parasite), it mounts an immune response. The immune system is made up of two parts: the innate, (general) immune system and the adaptive (specialized) immune system. These two systems work closely together and take on different tasks. Innate and adaptive immunityInnate immunity is the first line of defense against infection. Innate immunity acts very quickly. Phagocytic cells such as macrophages, neutrophils, barriers such as skin and a variety of antimicrobial compounds synthesized by the host all play significant roles in innate immunity. The closed surface of the skin and of all mucous membranes already forms a physical barrier against germs, which protects them from entering. The innate immune system activates special immune system cells and proteins if germs get past the skin and mucous membranes and enter the body. Adaptive immunity is the specific component of the immune system unlike the uniformity in all members of a species of the innate immunity. It specifically targets the type of germ that is causing the infection. Adaptive immunity has a high degree of specificity with the remarkable property of memory. It may take a few days for the adaptive immune system to respond the first time it comes into contact with the germ, but the next time the body can react immediately. The adaptive immune system is made up of T lymphocytes, B lymphocytes, and antibodies.T and B lymphocytesT lymphocytes (T cells) are the white blood cells produced in the bone marrow by the process of hematopoiesis. T cells mature in the thymus and during its maturation they express a unique antigen binding molecule called T-cell receptor, on their membrane. There are two well-defined subpopulations of T cells i.e, T helper cells (TH) cells and T cytotoxic (TC) cells. TH cells and TC cells can be distinguished from each other by the presence of either CD4 or CD8 membrane glycoproteins on their surface. T cells displaying CD4 function as TH cell and T cells displaying CD8 function as TC cells. T cell receptors can only recognize antigen that is bound to cell-membrane proteins called major histocompatibility complex (MHC) molecules. When a naive T cell recognizes and interacts with an antigen-MHC molecule complex, It is proliferated and differentiaites into memory T cells and effector T cells. There are two types of MHC (major histocompatibility complex) molecules: class I MHC molecules which are expressed by nearly all nucleated cells and class II MHC molecules which are expressed by only antigen presenting cells. When a T-helper cell (TH cell) encounters an antigen-MHC class II molecule complex, it gets activated and becomes an effector cell which secrets various growth factors known as cytokines. The secreted cytokines activate B cells, TC cells, macrophages and other cells that take part in immune response. B lymphocytes mature within the bone marrow and they express a unique antigen-binding receptor on their membrane. B-cell receptor is a membrane-bound antibody molecule. When a naive B cell first encounters the antigen that matches its membrane bound antibody, the antigen-antibody binding happens which causes the cell to divide rapidly into plasma B cells and memory B cells. Memory B cells have longer life span than plasma B cells. Plasma cells live for only few days and secret enormous amount of antibody during this short life span. Secreted antibodies are the major effectors of humoral immunity. AntibodiesAntibodies are compounds of protein and sugar that circulate in the bloodstream. They are created by the immune system to fight germs and foreign substances. Antibodies are produced by B lymphocytes. Antibodies can quickly detect germs and other potentially harmful substances, and then attach to them. Antibodies can neutralize germs, activate other immune system cells by attaching to their surfaces and activate proteins that help in the immune system response. Antibodies are glycoproteins that consist of twi identical heavy polypeptide chains and two identical light polypeptide chains. Each heavy chain is joined with the light chain by disulphide bonds. Humoral and cell mediated immunityImmune responses can be of two types i.e, humoral immunity and cell-mediated responses. Humoral immunity is mediated by macromolecules found in extracellular fluids such as secreted antibodies, complement proteins, and certain antimicrobial peptides. Humoral immunity is also referred to as antibody-mediated immunity. Antibodies are synthesized and secreted by plasma cells that are derived from the B cells of the immune system. An antibody is used by the acquir
