Body Lines Of Defense - Types, Examples (2023)

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Table of Contents

  • Defense Systems
  • 1. First Line of Defense
  • 2. Second Line of Defense
  • 3. Third Line of Defence (Adaptive Immune System)
  • Immunity Disorders
  • References
  • The immune system is a sophisticated network of particular immune cells and proteins that operate in concert to defend the body against external invaders and noxious environmental toxins.
  • Antigens are foreign chemicals that provoke an immunological response. Self-antigens can activate the immune system under particular conditions, such as in autoimmune illnesses, resulting in the destruction of the body’s biological components.
  • The immune system can generally be stimulated to produce two types of immune responses: nonspecific response (innate immunity) and specific adaptive response (acquired immunity).
  • Viruses, bacteria, and fungus comprise the human body’s three principal lines of protection against outside invaders.
  • The three lines of defence of the immune system include physical and chemical barriers, non-specific innate responses, and particular adaptive responses.

Defense Systems

  • Innate responses are the same regardless of the number of times a pathogen is exposed, but acquired responses improve with repeated contact to foreign particles.
  • Natural killer (NK) cells and phagocytic cells such as neutrophils, monocytes, and macrophages are utilised in innate responses. Adaptive responses, on the other hand, entail antigen-specific B and T cells or antigen-presenting cells (APCs).
  • Instinctive responses respond promptly to a foreign attack, whereas adaptive responses take longer.
  • Non-specific refers to the fact that innate reactions fight against all invaders, while acquired responses fight against specific categories of invaders.
  • The first and second lines of defence compose the inborn reaction, while the acquired response contains the third line of defence.
  • Essentially, there are three sorts of defensive lines:
    1. First line of defense
    2. Second line of defense
    3. Third line of defense

1. First Line of Defense

  • The first line of defence (or external defence system) consists of physical and chemical barriers that are constantly ready to defend the body against infection. These include your skin, tears, mucus, cilia, stomach acid, urine flow, “friendly” bacteria, and neutrophils, which are white blood cells.
  • Pathogenic (disease-causing) bacteria must breach this initial barrier. If this defence is breached, your body’s second line of defence is engaged.
  • Surface barriers that prevent viruses from entering the body are the body’s principal defence against infectious disease.
  • These surface barriers include intact skin and mucous membranes (which preserve external boundaries) (protect internal boundaries).
  • The skin and mucous membranes both secrete chemical substances that inhibit the growth of microorganisms on their surfaces.
  • If infections cannot penetrate the host, they cannot interfere with normal physiological activities and cause disease.

Components of First Line of Defense

Physical Component

It is the physical barrier that prevents pathogens from entering an area. It is composed of dead skin cells on the surface and mucus within the cell wall.

  • Skin – The tough layers of dead cells on the surface of the skin are constantly regenerated. However, the weak points lie in between, especially as the temperature outside drops, when they become slightly more permeable.
  • Mucus – Mucus traps foreign particles and transports dead cells away from the mucous membrane. It also conceals chemicals and shelters beneficial flora to repel invaders.

Chemical Component

It comprises of a range of proteins that attach to and destroy bacterial cell walls. These wall features are exclusive to bacteria, making them an attractive target that may be assaulted without endangering the health of tissue cells. There are basically three types:

  • Antibacterial Enzyme – Antibacterial substances such as secretory IgA and lysozyme are present in human saliva and tears. The function of these enzymes is to attack peptidoglycans (found in the cell walls of bacteria). Lysozyme is abundant in numerous secretions, including human milk, mucus, and egg white, in addition to saliva and tears. IgA (Immunoglobulin A) is crucial for mucosal immunity. Mucosal linings produce more IgA than all other antibody types combined.
  • Pore-forming Peptide – Antimicrobial peptides are an abundant and diverse set of chemicals produced by a number of invertebrate, plant, and animal tissues and cell types. Their amino acid makeup, amphipathicity, cationic charge, and size allow them to bind to and generate pores in bilayer membranes.
  • Gastric Acid – pH of less than 2.0 normally kills all bacteria, however this pH value is rarely maintained for an extended period of time, particularly during meal ingestion, when the majority of germs enter the stomach. Experiment demonstrates that many microorganisms cannot survive in an environment with a pH of 1.0 or 2.0, however all bacteria examined were able to thrive at pH 4.0, which is the pH level of the mucus layer. In the stomach’s acidic environment, bacteria can evade attack by adhering to food, neutralising stomach acid, or stopping the stomach from secreting additional acid. The H. pylori bacteria cause disorders such as gastritis and ulcers by moving in a screw-like manner with their distinctive flagella and transforming into a nonadherent condition to prevent adhesion to the mucus layer.
  • Gastric juice – Gastric juice (pH 2-3) is extremely acidic and destroys microorganisms that enter the stomach through the oral cavity or nasal cavity.
  • Urine – Urine flow, acidic in pH, kills microorganisms and exits the urethra directly.
  • Serum – Serum (unsaturated fatty acids) reduces water loss and inhibits microbial growth, but it also contains substances that provide nourishment to certain microorganisms.
  • Cerumen or earwax – Cerumen or earwax includes fatty acids, reducing the pH to between 3 and 5. It protects the auditory canal from external particles, such as microorganisms.

Biological Component

  • Each area of the gut generates a nourishing environment for precisely the type of bacteria that it needs. For example, the gut cells would provide a certain form of sugar for a particular bacteria when a signal is received from them.
  • In response, the bacteria would break down hard-to-digest food, add capillary branches, release antibiotic chemicals (with exemption for themselves), fill the space to block out undesired bacteria, manufacture vitamin K and other delights for the intestinal cells.
  • The bacteria in each site build a micro-ecosystem (called microbiome) with the members link together and interact with the environment on or in the host.
  • The microbiome acts quite similar to the macroscopic ecology, which exists in equilibrium even under tiny perturbation, except that it may respond swiftly to change and varied composition could perform the same function (to the host) (to the host).

2. Second Line of Defense

  • The second line of defence against infection are the non-specific cellular and molecular responses of the innate immune system.
  • The second stage of the innate immune system consists of cells and proteins that assault invaders. Innate defences are vague. In other words, no matter what infection your body is battling, the same response happens and the same cells and proteins are at work.
  • These defences do not differentiate between different types of pathogen and behave the same manner upon every infection.
  • Phagocytic leukocytes travel to infection sites and consume foreign substances (dendritic cells then give antigens to lymphocytes) (dendritic cells then present antigens to lymphocytes).
  • Inflammatory reactions enhance capillary permeability in infected areas, recruiting leukocytes but leading to localised edoema.
  • Antimicrobial proteins (such as cytokines and complement proteins) regulate immunological activity throughout the body.
  • Fever boosts body temperatures to activate heat-shock proteins and limit microbial growth and propagation.

Phagocytes (eating-cell) in 2nd line of defense

  • Monocytes – They exit the blood artery and develop into macrophages and one type of dendritic cell (messengers between the innate and the adaptive immune systems).
  • Macrophages -They are big, mobile, phagocytic cells that consume foreign matter, dead cells, and cellular detritus.
  • Neutrophils (neuter = neutral, phil = loving in Greek) – Neutrophils are the most numerous white blood cells. They enter sick tissue by squeezing past capillary walls, where they kill and consume intruders.
  • Eosinophils (eosin = red dye) – Eosinophils are a rare form of white blood cell, but their quantity increases dramatically in certain diseases, particularly illnesses caused by parasitic worms. They are cytotoxic, releasing their granules’ contents on the invaders.
  • Basophils – Basophils are likewise uncommon, and their quantity increases after an infection. They leave the blood vessel to converge and aggregate at the site of infection or other inflammation, where they release the contents of their granules and other antimicrobial compounds that play a significant role in certain allergic responses, such as hay fever and swelling in insect bites.
  • Mast (masto = breast) Cells – These granular-surfaced cells promote blood artery permeability by secreting histamine. This results in swelling, redness, and the attraction of additional inflammatory cells to the site of release.

Mechanism in 2nd Line of Defence

  • Blood transports phagocytes regularly throughout the body. By squeezing through the spaces between the capillary cells, they can choose to depart the blood vessels. Once there, they can either settle down or continue patrolling the tissue gaps.
  • The sensors of phagocytes can detect bacterial cell walls as well as waste particles or chemicals from dying or stressed human cells.
  • Once the indications for a bacterial invasion are identified, the phagocytes crawl to the source (much in common with the embryonic cell migration). They would secrete more signalling molecules for reinforcements, release a combination of very toxic compounds, and attempt to engulf and destroy the bacteria upon reaching the issue location.
  • The poisons are so deadly that they also cause significant harm to normal human tissue. The outcome is a local accumulation of redness and heat caused by an increase in blood flow, swelling from fluid and collecting cells, and pain from damaged nerve endings. In the centre of the inflammation is a region of pus that is predominantly composed of phagocytes, dead bacteria, and dead human tissue.
  • Excessive aggression is induced when the receptors detect signals from stressed and dead human tissue, such as following an injury. To ensure that any sickness is stopped in its tracks, every living thing in the region is wiped out.

3. Third Line of Defence (Adaptive Immune System)

  • The third stage of your immune system is comprised of cells specifically designed to eliminate the invading bacteria.
  • Lymphocytes that create antibodies to specific antigenic fragments serve as the ultimate line of defence against infection.
  • Each B cell creates a unique antibody, and the human body contains millions of diverse B cells capable of detecting various antigens.
  • Helper T cells control B cell activation, ensuring that antibodies are only made in large quantities at the proper periods.
  • After activation, both B and T cells will develop into memory cells, conferring long-term protection to a specific disease.
  • B and T lymphocytes are the two primary types of specialised white blood cells involved in the adaptive immune response. B cells exist within the blood. Their primary purpose is to develop into cells that create antibodies to combat antigens (foreign invaders) that enter the body. To achieve this, they collaborate with T cells.
  • Dendritic cells look for T cells within the lymph nodes. Your body produces millions of unique T cells. Each T cell subtype can recognise a distinct infection. This means that your body can battle practically any intruder, including those it has never encountered before!
  • In the lymph nodes, the T cells are fully developed, but they have never faced the virus they are designed to combat. Essentially, these cells are asleep. The function of dendritic cells is to arouse and transport immune cells to infections.
  • Different types of T lymphocytes perform distinct functions:
    • Memory T cells recall previously encountered infections. They assist your body in launching a speedier, more effective defence the following time.
    • Virus-infected cells in your body are eliminated by cytotoxic (“cell-killing”) T lymphocytes.
    • T helper cells assist other cells, such as B cells, frequently by producing cytokines. These proteins bind to other cells and instruct them on how to enhance the immune response. For instance, a cytokine may stimulate a B cell to produce antibodies against an invading infection. T helper cells are one of the most beneficial types of T cells when dealing with a bacterial infection caused by a cut finger.
    • T regulatory cells serve as the adaptive immune system’s police force. Once the pathogen has been eliminated, they cease the attack begun by other immune cells. This prevents the immunological response from becoming uncontrolled.

Immunity Disorders

The Immune System can be inefficient if it fails to identify aberrant cells as malignant cells, or it can be overprotective and create other issues.

(Video) First line of Defence | Physical & chemical barriers |



  • Hypersensitivity of the immune system to relatively harmless environmental antigens – the immune system reacts to an external material that it would ordinarily ignore – allergy kinds (food, dust, mould, seasonal), symptoms, and signs (skin rash, itching, red bumps, sneezing).


  • An obstructive pulmonary disorder characterised by recurrent spasms of bronchial wall muscles followed by edoema and mucus production that makes breathing difficult – it causes the airways of the lungs to swell and narrow, resulting in wheezing, shortness of breath, chest tightness, and coughing.
  • Extrinsic asthma, often known as allergic asthma, is more prevalent (90% of all cases) and typically begins in childhood.
  • Intrinsic asthma accounts for around 10% of all cases. It commonly manifests after the age of 30 and is typically unrelated to allergies. Inhaler containing drugs such as albuterol to open airways.

Autoimmune Disorder

  • A disorder in which the immune system erroneously targets and destroys healthy human tissue – more than 80 different types – because the immune system cannot distinguish between healthy body tissue and antigens.
  • Normal bodily tissues are destroyed as a result of an immunological reaction. This response is a hypersensitivity reaction comparable to the allergic response.
  • Addison’s disease, Celiac disease – (gluten-sensitive enteropathy), Graves disease, Hashimoto’s thyroiditis, Multiple sclerosis, Myasthenia gravis, Pernicious anaemia, Rheumatoid arthritis, Systemic lupus erythematosus, and Type I diabetes are examples of autoimmune (or immune-related) conditions.


  • AIDS (acquired immune deficiency syndrome) is the last stage of HIV disease, which causes severe damage to the immune system due to infection with the human immunodeficiency virus (HIV).

Tissue Rejection – Foreign MHC Proteins

  • White blood cells recognise the body’s tissues by searching for a set of antigens on the surface of each cell.
  • The most essential of them constitute the main histocompatibility complex (MHC).
  • Self-antigens – Protein markers of the major histocompatibility complex (MHC) – Two groups
    • Class I MHC markers are expressed on all cell types except for RBCs.
    • Class II MHC molecules are expressed on mature B-cells, some T-cells, and antigen-presenting cells.
  • Doctors screen the MHC of potential organ donors to determine the greatest match.

Blood typing problems

ABO System RBC surface membranes contain proteins that serve as antigens in some receivers.

  • Blood type A contains solely A antigens.
  • Blood type B contains solely B antigens.
  • Blood type AB has both A and B antigens.
  • Blood type O is devoid of both A and B antigens.
  • Blood plasma contains antibodies specific for blood types that are not present.
  • Exposure to foreign blood antigens causes agglutination or clustering of RBCs, halts blood circulation, and causes RBCs to explode.

Rh System problems

  • Another significant antigen utilised in blood type matching.
  • Rh positive individuals have Rh factor on the RBC membrane; Rh negative individuals lack the Rh factor protein.
  • Individuals who are Rh-negative do not have antibodies to the Rh factor, but develop immunity when exposed to it.
  • HDN can arise when the mother is Rh-negative and the newborn is Rh-positive.
    • Mother is only “inoculated” with a small amount of baby’s blood (and Rh protein) before delivery.
    • The mother develops antibodies that are small enough to cross the placenta and are capable of destroying the baby’s red blood cells; the mother receives a booster at each birth; hence, the risk to subsequent infants increases.
    • The problem is remedied by administering anti-Rh antibodies to the mother, generally after the birth of the baby, which attack any of the baby’s RBCs remaining in the mother’s blood before the mother can form antibodies.





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