The immune response is the protective response against the invading microorganism shown by the immune cells. When a foreign cell enters the body, the body fluid, lymph, takes these cells to the lymph nodes. The lymph nodes are the filter-like organs scattered in several parts of the body. The lymph nodes contain all the cell components of the immune system-B-cells, T-cells, and macrophages. The macrophages engulf the pathogen and are digested by the enzymes present in the lysosomes of the macrophages. These digested components of the pathogens are presented to the lymphocytes. Then a number of cellular mechanisms occur and a number of substances are secreted as a result of the immune response, which is based on the nature of the antigen (the degraded product of the pathogen). Basically, there are two types of immune response, which is dependent on two cellular systems: the humoral or circulating antibody system, (B-cells immunity), and cell-mediated immunity, (T-cells immunity).
Both immune responses work by identifying antigens (foreign proteins or polysaccharides) either as part of a virus or bacterium or as a partially degraded byproduct. Both systems also recognize human antigens not made by the individual resulting in graft rejection.
The humoral antibody system (B-cell response) produces secreted antibodies (proteins), which bind to antigens and identify the antigen complex for destruction. Antibodies act on antigens in the serum and lymph. B-cell-produced antibodies may either be attached to B-cell membranes or free in the serum and lymph. The cell-mediated system acts on antigens appearing on the surface of individual cells. T-cells produce T-cell receptors, which recognize specific antigens, bound to the antigen presenting structures on the surface of the presenting cell.
Humoral-Antibody System: B-cells
Each B-lymphocyte, or B-cells produces a distinct antibody molecule (immunoglobulin or Ig). Over a million different B-lymphocytes are produced in each individual. Thus, each individual can recognize over a million different antigens. The antibody molecules are glycoproteins in nature and each one is composed of two copies of two different proteins. There are two copies of a heavy chain, over 400 amino acids long, and there are two copies of a light chain, over 200 amino acids long. There are five different kinds of antibodies. They are IgG, IgM, IgA, IgD, and IgE. Each antibody molecule can bind two antigens at one time, thus, a single antibody molecule can bind to two viruses, which leads to clumping. When a new antigen comes into the body, it binds to the B-cell, which is already making an antibody that matches the antigen. The antigen-antibody complex is engulfed into the B-cell and partially digested. The antigen is displayed on the cell surface by a special receptor protein (MHC II) for recognition by helper T-cells. The B-cell is activated by the helper T-cell to divide and secrete antibodies, which circulate in the serum and lymph. Some B-cells become memory cells to produce antibody at a low rate for a long time (long-term immunity) and to respond quickly when the antigen is encountered again. The response is regulated by a class of T-cells called suppressor T-cells.
Cell-Mediated System: T-cells :
T-cells mature in the thymus, which is why it is called a T-cell. A large number of different kinds of T-cells, each producing a different receptor in the cell membrane, are present in the system. Each receptor is composed of one molecule each of two different proteins. Each receptor binds a specific antigen but has only one binding site. Receptors recognize only those antigens, which are "presented" to it by another membrane protein of the MHC type (major histocompatibility complex). T-cell receptors recognize antigens presented by B-cells, macrophages, or any other cell type. T-cells, B-cells, and macrophages use MHC-II receptors for presentation; all other cells use MCH-I (responsible for most of tissue graft rejection). When a T-cell is presented with an antigen, its receptor binds to the antigen and it is stimulated to divide and produce helper T-cells to activate B-cells with bound antigen, suppressor T-cells to regulate the overall response, and cytotoxic "killer" T-cells to kill cells with antigen bound in MHC-I.
Tags: Bio Technology, Bio Genetics, Immune System
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