# Structural Biochemistry/Cell Signaling Pathways/Immune System

The Immune System
    The world is teeming with such a wide variety of parasites, bacteria, and viruses, and many of these can be potentially very harmful or devastating to the human body. However, humans have a remarkable ability to defend against unfamiliar organisms, and they owe their immune systems to this defense. The immune system is the body's way of identifying between itself and what is foreign. The human body can make more than 10$^8$ different antibodies and 10$^{12}$ T-cell receptors. These all represent a different foreign invader the body can bind to and begin to destroy.
The Immune System Adapts
    The human immune system can essentially take on a limitless number of different pathogens. It does this by transforming its immune cells. This huge degree of variability and flexibility is rooted in the principles of evolution. Generations of reproduction and selection has resulted in this powerful system, which is composed of two interrelated systems.
    The humoral immune response utilizes antibodies, which are secreted by plasma cells in B lymphocytes. Antibodies, also known as immunoglobulins, are soluble proteins that recognize and bind to foreign invaders, marking them for destruction. The site on the foreign invader to which the antibody binds is referred to as the epitope or antigenic determinant. If the body gives an immune response when the antibodies bind the epitopes of the invaders, then the invader molecule is an immunogen.
    The cellular immune response utilizes cytotoxic T lymphocytes, which are also referred to as killer T cells. These cells destroy cells that have foreign markers on their surfaces. Cytotoxic T lymphocytes have special receptors on their surfaces that mediate this immune response.
    Helper T lymphocytes are a specific kind of T cell that are involved in both the humoral and cellular immune response. They stimulate the specialization and propagation of B lymphocytes and cytotoxic T lymphocytes.
The Humoral Immune Response
    Immunoglobulin G is the most common antibody in serum. Because it has two binding sites, it is able to cross-link multiple antigens. It also has segmental flexibility, or flexibility in regions of polypeptide that allow angle variation. This allows the antibody to bind an antigen in multiple binding sites.
    Immunoglobulin M is the first kind of antibody to enter serum after the body is exposed to an antigen. It is able to bind many multivalent antigens that immunglobulin G is unable to. Immunoglobulin A is the major antibody in saliva, tears, and other external bodily secretions. It is first in the body's defense when it comes to bacteria and viruses. Immunoglobulin E protects against parasites but is also responsible for allergic reactions.
    Antibodies have domains, or common sequences that adopt the immunoglobulin fold structure. Immunoglobuiln G has 12 immunoglobulin domains that consist of a pair of antiparallel beta sheets held together by disulfide bonds and hydrophobicity. There are two particularly important structures in the structure. Firstly, at one end there are three hypervariable loops that provide the mechanism for generating antibodies and T-cell receptors. The loops allow antibodies to bind to specific molecules. Secondly, the amino and carboxyl groups at the ends of the structure allow domains to form chains together. The immunoglobulin domains together result in the immunoglobulin fold structure, a structure so common that more than 700 genes encode for proteins that have at least one.
    The humoral immune system is also referred to as the antibody-mediated system. Firstly, macrophages, a special blood cell, ingest the foreign invader through phagocytosis. They digest the invader and then display bits of it on their cell membranes. Helper-T cells, upon recognizing this, multiply quickly. This is referred to as the activation phase. The helper-T cells then contact B-cells through chemical signals, and the B-cells also begin to multiply quickly. This is called the effector phase. The B-cells divide and their daughter cells may be plasma cells, which produce great amounts of antibodies to signal the destruction of antigens, or B memory cells, which last in the body, keeping it immune to that same antigen for years to come.
The Cellular Immune Response
    Many viruses and mycobacteria are not easy to detect. Appropriate T-cell receptors can recognize harmful foreign invaders, and recognize when cells should undergo apoptosis, or cell death. This prevents damage to surrounding cells. The cellular immune response is also referred to as the cell-mediated system. It involves cytotoxic T cells that recognize when other cells have been infected by a foreign invader. As a result, they ingest and destroy the infected cell. Cytotoxic T cells are also vital to the immune system in that they can recognize and destroy larger invaders such as parasites.
    Like B cells, T cells too produce memory cells. The body's combination of T and B memory cells enables it to more quickly respond should the same foreign invader enter the body. This is referred to as a secondary response.
So What?
    The immune system is obviously vital in maintaining the body's health in a world full of foreign invaders, bacteria, and viruses. However, the body's ability to create T and B memory cells provide uses in the field of health. Today, more and more shots have been created to make the body immune to the different diseases throughout the world. This holds great potential for the diseases that are still rampant in society today. Perhaps one day there will be an immunization for cancer.
Source: Berg, Jeremy and Stryer, Lubert. Biochemistry: Fifth Edition. United States of America: W.H. Freeman and Company, 2002.