Your immune system is your body’s first line of defence against infection and just like any other part of your body, it can be affected by how well you look after it.

Innate vs Adaptive Immunity

The purpose of the immune system is to protect the body from harmful cells or debris entering from the outside world. Where those objects do find their way into the body, biological processes exist to minimise the damage those foreign objects do to the body. The first line of defence is termed innate immunity which encompasses protective measures that the body has built into its biological structure and chemical processes for destroying harmful cells. Further, this is aided by the adaptive immune response in which the body can develop an immunological memory to combat harmful cells that it has encountered previously.

A white blood cell ingesting disease-causing bacteria
Image: A white blood cell ingesting disease-causing bacteria.

Both the innate and adaptive immune systems have evolved to work in partnership to protect the body from harmful pathogens. Pathogen refers to any potentially damaging smaller organisms including microbes, for example, bacteria, viruses, fungi, and parasites, for example, tapeworms. For those macromolecules, cells and organisms which the body recognises as being harmful, ie to which it initiates an immune response, they can be referred to as antigen. It is also worth noting that the body must be able to recognise its own cells as being different from those specific antigens such that a response is not triggered against itself and this is termed immunological tolerance.

Video: How Pathogens Are Ingested By Phagocytes

The innate immune system has no memory of the antigen but reacts rapidly to it. At the body’s surfaces the skin acts as an impermeable barrier to most pathogenic organisms and is covered in sweat which is slightly acidic to inhibit growth of bacteria. Secondly, mucus lining the respiratory tract, reproductive tract and the gut traps microbes on its sticky surface and cilia in the lungs push mucus and particles toward the throat where it can be cleared or swallowed.

Lysozyme and interferons make up the chemical component of the innate immune response; the first is an enzyme which splits the chemical bonds holding together molecular components of bacterial cell walls; the second is a special group of signalling proteins which are manufactured when a cell becomes infected with a virus with the property of damaging viral mRNA such that protein translation is inhibited in the affected and neighbouring uninfected cells to create a barrier of cells which cannot manufacture new virus particles.

A human lymphocyte
Image: A human lymphocyte.

The innate immune system is supported by lymphocytes which are manufactured in haematopoietic stem cells of bone marrow. They serve the following functions: phagocytosis, in which they are capable of digesting potentially harmful material; cytotoxicity, where the cell membrane of a pathogen or infected host can be damaged to kill infectious organisms; generating inflammation around the site of an infection to attract other lymphocytes to the area to combat pathogens, a process termed chemotaxis.

Video: How Lymphocytes Fight Infection

When a certain kind of pathogen enters the body for the first time, a primary adaptive response occurs. This is barely detectable for the first 7-10 days but activity peaks within 2-3 weeks. If the same pathogen is encountered in the body a second time, an enhanced secondary adaptive response occurs – this response develops sooner, lasts longer and displays greater levels of activity than the primary response. Thus, the adaptive immune system delivers a specific response which adapts based on previous encounters with antigen to combat recurring infection.

Each type of pathogen is recognised by its ‘marker’, commonly referred to as its epitope which forms the immunological memory. Antigen specificity refers to the ability for the immune system to distinguish between one kind of antigen and another by recognising and responding to epitopes on the surface of pathogens. The antigen binding site is an area in which the epitope and the antigen receptor interact with an exactly complementary shape protruding from a small lymphocyte’s surface.

The lymphatic system is the main territory for the adaptive immune system response and is responsible for defending the body against invading organisms; collecting and returning interstitial fluid to the blood; absorbing lipids from the digestive tract.

Immune System

The small lymphocytes are composed of B and T cells. B cells mature in bone marrow, producing antibodies, proteins designed to bind to and destroy an antigen. After contact with antigen, B cells undergo clonal expansion and differentiate into either memory B cells or plasma cells. Antigen receptors on B cells are Y-shaped with 2 arms protruding outwards with a binding site at the tip of each arm and become activated by an antigen to provide one of 5 classes of antibodies – IgG, IgA, IgM, IgD, or IgE.

T cells mature in the thymus by undergoing clonal deletion in which specialised cells in the thymus present self-epitope cells on their surface membranes; as the juvenile T cells pass through, any with antigen receptors that bind to the self-epitopes are given a chemical signal that causes them to undergo apoptosis and die. T cells that do not bind to the self-epitopes emerge from the thymus as mature T cells and this provides self-tolerance.

Whilst the innate immune system reacts to any antigen using physical barriers and chemical processes involving interferons, lysozyme and phagocytosis, the adaptive immune system provides a more specific targeted response which grows stronger after an antigen has been encountered.

Marc DinardoWritten by Marc Dinardo

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