Basic concepts in Modern Immunology

Immunology is the study of the various mechanisms of host defenses against infections, infectious diseases and neoplasm, together with the undesirable consequences of immune interactions such as inflammation, autoimmunity, and hypersensitivity reactions.

Pathogens are organisms that invade our body to cause disease. They are five types of human pathogens;

  1. Bacteria
  2. Fungi
  3. Viruses
  4. Protozoa
  5. Worms (helminthes)

Bacteria, fungi, and viruses are known as microbes, whereas, protozoa and worms are called parasites. Pathogens invade the body when we drink contaminated water, breathe contaminated air, eat contaminated food, or have contact with it through our skin. Pathogens enter the human body to cause disease.

Most pathogens enter the body but, live outside body cells (in body fluids), and are called extracellular pathogens, but, some pathogens in an attempt to escape attack from immune system, invade body cells to live right inside host’s body cells; such pathogens are called intracellular pathogens. Viruses are the main intracellular pathogens

Immune system is the collection of cells and tissues that help the body to resist infections. In humans, the immune system is categorized into two functional branches: innate and adaptive immune systems.

It is important to note that both innate and acquired branches of immune systems are not entirely separate from each other. Both systems cooperate between themselves to detect, attack, and eliminate foreign substances from the body.

Innate immune system constitutes physical barriers, chemical barriers, immune cells, and other immune reactions that remain unchanged upon subsequent exposure to the same antigen.

The main mechanism utilized by innate immune system in killing harmful foreign agents is phagocytosis and the generation of toxic substances that damage membrane proteins of invading microbes.

Acquired immune system and immunity: innate immune system alone is unable to eliminate pathogenic microbes; acquired immune system offers a more effective attack against pathogens and also keeps an immunological memory of all pathogens it encounters. Acquired immune system provides the body with immunity.

Lymphocytes (both B and T-lymphocytes) mediate acquired immunity and also produce an immune response.

Immunity: immunity in is the state of being resistant to foreign antigens and microorganisms. In specific terms, innate immune system is unable to provide ‘immunity’ against any particular foreign antigen. The antimicrobial actions of innate immune system are non-specific responses that help to prevent the spread of the infection and also to trigger the acquired immune system to respond.

It is the acquired immune system that provides the body with immunity against any foreign antigen. This is because the acquired immune system is antigen-specific in response, and also keeps immunological memory of all foreign antigens it encounters.

It is important to note that the hallmarks of immunity are specificity and memory; and both are absent in the innate immune reactions and responses. An individual has immunity against a foreign antigen, only, when his system has developed an immunological memory against the same antigen.

Humoral and cell-mediated (cellular) immunity

The defense provided by the body against pathogens (immunity) is divided into

  1. Humoral and
  2. Cellular immunity.

Humoral immunity is mediated by antibodies. Antibodies are produced by plasma cells, which are themselves derived from B-lymphocytes. B-lymphocytes, when activated, proliferate, enlarge and transform into plasma cells. Antibodies are soluble substances which bind to specific antigens tagging the antigens for destruction.

Specificity of antibodies: When foreign antigens invade the body, the body produces antibodies against the foreign antigen. One of the most important properties of the antibodies produced is that they bind only with the inducing antigen and are unable to bind with other antigens in the body. This property is called specificity of antibodies.

Cellular immunity is mediated by T-lymphocytes. A type of T-lymphocytes called Cytotoxic T-lymphocytes directly attack and destroy cells that carry antigens specific for their receptors.

Humoral immune responses particularly combat extracellular pathogens whereas cell-mediated immune responses particularly combat intracellular pathogens.

Immune response: The series of reactions of cells of the immune system to a foreign antigen is called an immune response. In normal immune function, only potential harmful foreign antigens trigger immune responses in the body. Foreign antigens that pose no threat to the system are normally ignored by the immune system.

A normal immune response starts with the recognition of a potential harmful foreign antigen in the body, followed by activating and mobilizing forces against the antigen, and ends with an attack that eliminates the antigen.

Immunological memory: At the end of all immune responses, after the invading pathogen has been completely eliminated, some lymphocytes differentiate into memory cells. Memory cell express receptors for the specific antigen that challenged the system. These cells can remain dormant and viable for decades. When the same individual comes in contact with the same antigen for a second time, the immune response will be more effective and rapid than the first one. The first immune response is called primary immune response whereas the second immune response is called secondary immune response.

An antigen is any substance that can activate the immune system and provoke it to produce antibodies. They are non-self markers that trigger the immune response. They trigger the formation of armies of lymphocytes.

There are many forms of antigens; some occur as small molecules in the atmosphere while others occur as protein molecules on the surface membranes of bacterial and viral cells. These non-self markers can either be expressed on the surfaces of pathogens; contained within pathogens; or rather exist on their own. Examples of antigens that exist on their own are dust particles and pollen.

 Self antigens, self tolerance and autoimmunity

Self-antigens are proteins that cover the surface membrane of all body (host) cells; some of these proteins may be foreign to other people and cause rejection during transplant and transfusion.

Self-tolerance is a term used to describe the ability of immune effector cells to recognize and tolerate self-antigens. Self-reactive immune cells are normally deleted from the pool of immune cells.

Autoimmunity (immunity against self) occurs when self-reactive immune effector cells find their way into peripheral blood, and attack self-antigens on host cells destroying host’s cells and tissues.

Antigen-presenting cells are phagocytic cells that engulf and digest foreign cells but leave the antigens intact; and couple these antigens to Major Histocompatibility Complex molecules before displaying the complex (the MHC-antigen complex) to lymphocytes that can recognize the antigens. Whenever a foreign cell invades the body, APCs take up the foreign cells, digest them and present their antigens to lymphocytes notifying them that a foreign antigen is present within the body. Well known antigen presenting cells are:

  1. Dendritic cells,
  2. Macrophages, and
  3. B-lymphocytes.

 Complement is a name given to a group of proteins that are involved in a series of reactions called complement cascade. In the absence of an infection, complement proteins circulate in plasma in inactive form. But, during infection, complement proteins become activated and undergo a series of reactions, called complement cascade, which yields many more products of complement.

The products of complement cascade participate in fighting harmful foreign agents by acting as opsonins, chemotaxins and chemical substances that directly cause lysis of pathogenic cells.

Complement proteins mediate pathogen-killing effects of both innate and acquired immune systems.

Cytokines: Cytokines are proteins secreted mainly by immune cells; though they are also produced by non-immune cells. They act as the immune system messengers to help regulate immune functions and responses.

Whenever you wonder about what stimulates immune cells to proliferate, differentiate, and mature; and what controls immune cells actions, movements, and lifespan, think cytokines.

Lectures on this topic continues here: Innate Defense Mechanisms I

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