© 2019 by Mark Street.

Immune System //

The Immune system is complex and much of its activities, at least in humans, is still not completely understood. The immune system works because the body is normally able to recognize "self" and "non-self." This means that the body is able to tell if an invader (virus, bacteria, parasite, or other another person's tissues) has entered it—even if you aren't consciously aware that anything has happened. Your body recognizes this invader and uses a number of different tactics to destroy it.

The immune system involves many parts of the body (Refer to Figure 1).

Figure 1 //

Source: www.aids.gov/

The immune cells originate from the bone marrow where some cells mature, whilst other cells mature in the thymus. When a foreign body enters the body the immune cells migrate via lymph fluid in lymph vessels to various organs within the body including the lymph nodes (lymph node swelling), appendix, areas within the gastrointestinal tract, the spleen and the tonsils and adenoids.

The components of the immune system include:

  • B lympohcyte cells: B cells originate from the bone marrow and then circulate around the body. B cells work chiefly by secreting substances called antibodies into the body’s fluids, flagging attack from other cells of the immune system. When a B cell encounters its triggering antigen (such as a virus or bacteria), it gives rise to many large cells known as plasma cells. Every plasma cell is essentially a factory for producing an antibody. Each of the plasma cells descended from a given B cell manufactures millions of identical antibody molecules and pours them into the bloodstream. 

 

  • Antibodies belong to a family of large molecules known as immunoglobulins. Different types play different roles in the immune defense strategy.

  • Immunoglobulin G, or IgG, 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, the secretions of the respiratory tract and the digestive tract—guarding the entrances to the body.

  • IgE, whose natural job probably is to protect against parasitic infections, is the villain responsible for the symptoms
    of allergy.

  • IgD remains attached to B cells and plays a key role in initiating early B-cell response. 

  • T Lymphocytes cells: T cells originate from the bone marrow and mature in the thymus gland (hence the name T cells). T cells do not recognise free floating antigens, instead binding to antigens bound to the surface of infected or cancerous cells. T cells contribute to immune defenses in two major ways: some direct and regulate immune responses; others directly attack infected or cancerous cells. Thus T cells functions include:

    • Coordinating immune responses by communicating with other cells (cytokines). Other T cells stimulate B cells to produce antibody, others call in microbe- gobbling cells called phagocytes, still others activate other T cells (T helper cells).

    • Killer T cells—also called cytotoxic T lymphocytes or CTLs—perform a different function. These cells directly attack other cells carrying certain foreign or abnormal molecules on their surfaces. 

  • Phagocyte cells: Phagocytes are large white cells that can swallow and digest microbes and other foreign particles. Monocytes are phagocytes that circulate in the blood. When monocytes migrate into tissues, they develop into macrophages. Specialized types of macrophages can be found in many organs, including lungs, kidneys, brain, and liver.

      Macrophages play many roles including acting as scavengers, ridding the body of worn-out cells and other debris. They display       bits of foreign antigen in a way that draws the attention of matching lymphocytes. 

 

  • Granulocytes: Granulocytes are immune cells that contain granules filled with potent chemicals, which allow the granulocytes to destroy microorganisms. Some of these chemicals, such as histamine, also contribute to inflammation and allergy.

      One type of granulocyte, the neutrophil, is also a phagocyte; it uses its prepackaged chemicals to break down the microbes it         ingests. Eosinophils and basophils are granulocytes that “degranulate,” spraying their chemicals onto harmful cells or microbes       nearby. The mast cell is a twin of the basophil, except that it is not a blood cell. Rather, it is found in the lungs, skin, tongue,             and linings of the nose and intestinal tract, where it is responsible for the symptoms of allergy.

  • Complement System: The complement system is made up of about 25 proteins that work together to “complement” the action of antibodies in destroying bacteria. Complement also helps to rid the body of antibody-coated antigens (antigen-antibody complexes). Complement proteins, which cause blood vessels to become dilated and then leaky, contribute to the redness, warmth, swelling, pain characteristic of an inflammatory response. The complement proteins bind together into a cascade, forming a cylinder and puncturing the cell. Phagocytic cells then 'mop' up the cell debris.

As noted previously, normally the immune system is able to identify foreign versus self. For many reasons unknown however, the immune system mistakenly attacks components of the body. This self-attack is termed autoimmune disease. Refer to Figure 2 which details the disease process (pathogenesis) of a group of vasculitis diseases called ANCA associated vasculitis (AAV). The diagram illustrates infection by the bacterium Staphylococcus aureus which in turn elicits the T helper cells to produce various chemicals (cytokines) for the recruitment of other immune cells including neutrophils, monocytes, and B cells. The neutrophils and monocytes contain proteins, namely proteinase 3 (PR3) and myleoperoxidase (MPO) which are expressed on the cell walls of neutrophils and monocytes, as well as released into the bloodstream upon T cell activation. It is at this point that the body becomes confused and resultantly produces antibodies to the PR3 and/or MPO. Consequently, the immune cells produce a range of infammatory chemicals including reactive oxygen species (ROS) resulting in the destruction and inflammation of the vascular cell wall (vasculitis). Frequently complexes of immune cells are deposited at the site (granulomas).

Figure 2 //
Video //

Refer to Video which provides an overview on the immune system and autoimmunity.

Source: www.aarda.org