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B CELL

B Cells

Immunoglobulins. View credit information.

B cells and T cells are the main types of lymphocytes. B cells work chiefly by secreting substances called antibodies into the body’s fluids. Antibodies ambush foreign antigens circulating in the bloodstream. They are powerless, however, to penetrate cells. The job of attacking target cells—either cells that have been infected by viruses or cells that have been distorted by cancer—is left to T cells or other immune cells (described below).

Each B cell is programmed to make one specific antibody. For example, one B cell will make an antibody that blocks a virus that causes the common cold, while another produces an antibody that attacks a bacterium that causes pneumonia. When a B cell encounters the kind of antigen that triggers it to become active, it gives rise to many large cells known as plasma cells, which produce antibodies.

• Immunoglobulin G, or IgG, is a kind of antibody that works efficiently to coat microbes, speeding their uptake by other cells in the immune system.
• IgM is very effective at killing bacteria.
• IgA concentrates in body fluids—tears, saliva, and the secretions of the respiratory and digestive tracts—guarding the entrances to the body.
• IgE, whose natural job probably is to protect against parasitic infections, is responsible for the symptoms of allergy.
• IgD remains attached to B cells and plays a key role in initiating early B cell responses.

Adaptive immune system

The adaptive immune system, also known as the specific immune system, is composed of highly specialized, systemic cells and processes that eliminate or prevent pathogenic growth. Thought to have arisen in the first jawed vertebrates, the adaptive or "specific" immune system is activated by the “non-specific” and evolutionarily older innate immune system (which is the major system of host defense against pathogens in nearly all other living things). The adaptive immune response provides the vertebrate immune system with the ability to recognize and remember specific pathogens (to generate immunity), and to mount stronger attacks each time the pathogen is encountered. It is adaptive immunity because the body's immune system prepares itself for future challenges.

The system is highly adaptable because of somatic hypermutation (a process of accelerated somatic mutations), and V(D)J recombination (an irreversible genetic recombination of antigen receptor gene segments). This mechanism allows a small number of genes to generate a vast number of different antigen receptors, which are then uniquely expressed on each individual lymphocyte. Because the gene rearrangement leads to an irreversible change in the DNA of each cell, all of the progeny (offspring) of that cell will then inherit genes encoding the same receptor specificity, including the Memory B cells and Memory T cells that are the keys to long-lived specific immunity. Immune network theory is a theory of how the adaptive immune system works, that is based on interactions between the variable regions of the receptors of T cells, B cells and of molecules made by T cells and B cells that have variable regions.

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Innate immune system

The innate immune system, also known as non-specific immune system and first line of defense,[1] comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner. This means that the cells of the innate system recognize and respond to pathogens in a generic way, but unlike the adaptive immune system, it does not confer long-lasting or protective immunity to the host.[2] Innate immune systems provide immediate defense against infection, and are found in all classes of plant and animal life.

The innate immune system is thought to constitute an evolutionarily older defense strategy, and is the dominant immune system found in plants, fungi, insects, and in primitive multicellular organisms.[3]

The major functions of the vertebrate innate immune system include:

    * Recruiting immune cells to sites of infection, through the production of chemical factors, including specialized chemical mediators, called cytokines.
    * Activation of the complement cascade to identify bacteria, activate cells and to promote clearance of dead cells or antibody complexes.
    * The identification and removal of foreign substances present in organs, tissues, the blood and lymph, by specialised white blood cells.
    * Activation of the adaptive immune system through a process known as antigen presentation.
    * Acting as a physical and chemical barrier to infectious agents.