MECHANISMS OF INNATE IMMUNITY

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The parts of the body and other biological/immunological processes that are responsible for innate immunity include the skin, mucous membranes, inflammation, phagocytic actions, mammary glands (secretary immunoglobulin A, sIgA and/or colostrum), cytokines and complements.

THE SKIN

The skin is an intact, tough and impermeable barrier that covers the human body and protects it from microbial invasion. When the integrity of the intact skin is breached or cut open as a result of burns, needle prick, insect bite, abrasions or wounds, the skin becomes access way through which infectious disease agents enters the body. The skin has a low pH, and in addition to this, it also secretes some antimicrobial substances such as fatty acids and lactic acids in sweats – which help to keep pathogenic microorganisms at bay. The dry nature of the skin coupled with the dense population of resident microflora on it help to keep foreign substances and pathogenic microbes in check or under control. The mechanical barrier of the intact skin, the presence of lysozymes (enzyme that dissolves the cell walls of some bacteria) and the acidic nature (pH 3-5) of the skin help to impede the growth of pathogenic microbes on the skin.  

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MUCOUS MEMBRANE

Mucous membranes are largely found in the nose, mouth, gastrointestinal tract (GIT), cervicovaginal tract, urinogenital tract and the respiratory tract where their main function is to trap pathogenic microorganisms and other foreign substances that tries to enter the body through any of these routes. Mucous is a biological secretion produced by membranes that line the inner surfaces of the body. They contain substances or compounds that are either bactericidal or bacteriostatic to invasive microbes. Lysozyme is a typical example of an antimicrobial compound secreted in mucous secretions. Microbial attachment to epithelial surfaces of the body is usually the first stage towards the development of an infection.

Thus, mucous secretions containing secretory antibodies help to prevent this microbial attachment from taking place. Invasive microbes trapped by mucous secretions of the body are removed by reflex actions such as sneezing, coughing and cilliary movements of the respiratory tract which drives pathogenic microbes upwards and outwards. The flushing mechanisms of the urine, tears and saliva also help to trap and remove microbes from the urinogenital tract, eyes and the mouth respectively. In the vagina of adult females, the mucous secretions from the vaginal wall contains normal flora (e.g., lactobacillus or lactic acid bacteria) which maintains acidic pH that prevent the colonization of pathogenic microorganisms in that part of the body. Also in the GIT, a low pH and anaerobic environment is maintained by the resident normal flora in order to inhibit and prevent colonization by pathogenic microbes.     

INFLAMMATION

Inflammation is the general response of the body to microbial invasion or tissue injury (e.g., wound). It is a complex series of immunological response (comprising both the humoural immunity and cellular immunity) that usually involves the accumulation of immune system cells and fluids around damaged body tissues or injuries. Microbial invasion elicits inflammatory response by causing the leakage of phagocytes and vascular fluids that contain serum proteins with antibacterial activity into the affected body area. This response helps to defend the body against infection and possible tissue damage. Cytokines and interferons (IFNs) are some of the chemical mediators released during an inflammatory response. The inflammatory response that occurs during microbial invasion or tissue damage is strong enough to prevent the spread of pathogenic microbes including pathogenic viruses, bacteria, and fungi from their portal of entry to other body sites.

PHAGOCYTIC ACTION

Phagocytic action is mediated by special types of cells known as phagocytes – which swim into action following microbial invasion of the body. Phagocytes are recruited in large numbers and they travel to the site of inflammation and move around in the blood circulation to eat up or engulf microbial cells and antigens. The process by which phagocytes ingest extracellular particulate materials (i.e., pathogenic microbes and foreign bodies) is known as phagocytosis. Phagocytosis is an important innate defense mechanism that is carried out by a variety of cells collectively called phagocytes. The main functions of these cells are to ingest, kill and digest whole pathogenic microorganisms that invaded the body.  

CYTOKINES

Cytokines are low-molecular weight molecules of the innate immune system which are released by leukocytes to help in the regulation of inflammatory response during microbial invasion. They are chemical mediators which play specific augmenting and regulatory roles in innate defense mechanism. Some examples of cytokines include interferons (IFNs), interleukins (ILs), and tumour necrosis factor (TNF). Cytokines also provide innate immunity against viruses and tumour cells in host cells. 

COMPLEMENTS

Complements are a series of serum proteins that play significant role in innate defense mechanism by catalyzing the killing of bacterial cells. They also facilitate the migration of white blood cells to sites of inflammation or infection in the body. This group of independent serum proteins which works as a system and in cascade is generally known as “complements” because they balance or assist the antibody response of the immune system in the face of microbial invasion. Complements are heat-labile glycoproteins found in blood plasma and serum, and they help bacterial opsonization by antibodies. Specifically, complements consist of nine (9) numbered components (i.e., C1 – C9) in humans; and these are enzymatically activated through a series of pathways that make up the complement system.

These numbered components of the complement are the individual proteins that make up the complements, and as earlier said, each is assigned a number from 1 – 9 to read C1, C2, C3, C4 and so on till C9. Each of these individual proteins of the complement system plays specific roles in both the innate and adaptive immune response. Complements are usually designated with the letter ‘C’, and this is usually followed by a number which designates the type. Complements are also present in tissue fluids and plasma. Complements are also known to help improve or augment the mechanism of the adaptive immune response since they are activated through antigen-antibody reaction. Complements play significant role in defending the host against infection and pathogen invasion – in that they help to mediate phagocytosis, bacterial lysis, opsonization and inflammation. 

MAMMARY GLANDS

The mammary gland (i.e., the breast in females) is an anatomical structure that provides a non-immunologically surface protective mechanism in humans. The flushing action of the milk that emanates from the breast helps to prevent the invasion of foreign bodies and pathogenic microbes. Also, the milk contains some substances with antimicrobial properties (e.g., lysozymes) that lyse bacterial cells including pathogens. In addition, the first milk from the mammary gland (known as colostrum) helps to initiate some level of immunity in newborns.

Further reading

William E.P (2003). Fundamental Immunology. 5th edition. Lippincott Williams and Wilkins Publishers, USA.

Stevens, Christine Dorresteyn (2010). Clinical immunology and serology. Third edition. F.A. Davis Company, Philadelphia.

Silverstein A.M (1999). The history of immunology. In Paul, WE (ed): Fundamental Immunology, 4th edition. Lippincott Williams and Wilkins, Philadelphia, USA.

Paul W.E (2014). Fundamental Immunology. Seventh edition. Lippincott Williams and Wilkins, USA.

Male D, Brostoff J, Roth D.B and Roitt I (2014). Immunology. Eight edition. Elsevier Saunders, USA.

Levinson W (2010). Review of Medical Microbiology and Immunology. Twelfth edition. The McGraw-Hill Companies, USA.

Berzofsky J.A and Berkower J.J (1999). Immunogenicity and antigen structure. In Fundamental Immunology, 4th edition., W.E. Paul, ed., Lippincott-Raven, Philadelphia. 

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