ImmunityInquiry question: How does the human immune system respond to exposure to a pathogen?Students:●      investigate and model the innate and adaptive

immune systems in the human body●      explain how the immune system responds after

primary exposure to a pathogen, including innate and acquired immunity

The Immune systemThe immune system is made up of special organs, cells and chemicals that fight infection (microbes). The main parts of the immune system are: white blood cells, antibodies, the complement system, the lymphatic system, the spleen, the thymus, and the bone marrow. These are the parts of your immune system that actively fight infection.

The immune system and microbial infection

The immune system keeps a record of every microbe it has ever defeated, in types of white blood cells (B- and T-lymphocytes) known as memory cells. This means it can recognise and destroy the microbe quickly if it enters the body again, before it can multiply and make you feel sick.

Some infections, like the flu and the common cold, have to be fought many times because so many different viruses or strains of the same type of virus can cause these illnesses. Catching a cold or flu from one virus does not give you immunity against the others. 

Parts of the immune system

white blood cells antibodies complement system lymphatic system spleen bone marrow thymus.

White blood cells White blood cells a are made in your bone marrow and are part of the lymphatic system. White blood cells move through blood and tissue throughout your body, looking for foreign invaders (microbes) such as bacteria, viruses, parasites and fungi. When they find them, they launch an immune attack. 

White blood cells include lymphocytes (such as B-cells, T-cells and natural killer cells), and many other types of immune cells. 

Antibodies Antibodies help the body to fight microbes or the toxins (poisons) they produce. They do this by recognising substances called antigens on the surface of the microbe, or in the chemicals they produce, which mark the microbe or toxin as being foreign. The antibodies then mark these antigens for destruction. There are many cells, proteins and chemicals involved in this attack. 

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The Lymphatic system

The lymphatic system is a network of delicate tubes throughout the body. The main roles of the lymphatic system are to:

manage the fluid levels in the body react to bacteria deal with cancer cells  deal with cell products that otherwise would result in disease or disorders absorb some of the fats in our diet from the intestine. 

The lymphatic system is made up of:

lymph nodes (also called lymph glands) -- which trap microbes lymph vessels -- tubes that carry lymph, the colourless fluid that bathes your body's

tissues and contains infection-fighting white blood cells white blood cells (lymphocytes).

Spleen The spleen is a blood-filtering organ that removes microbes and destroys old or damaged red blood cells. It also makes disease-fighting components of the immune system (including antibodies and lymphocytes).

Bone marrow Bone marrow is the spongy tissue found inside your bones. It produces the red blood cells our bodies need to carry oxygen, the white blood cells we use to fight infection, and the platelets we need to help our blood clot. 

Thymus The thymus filters and monitors your blood content. It produces the white blood cells called T-lymphocytes.

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Innate and Adaptive Immunity

The immune system is typically divided into two categories--innate and adaptive--although these distinctions are not mutually exclusive.

Innate immunity Innate immunity refers to nonspecific defence mechanisms that come into play immediately or within hours of an antigen's appearance in the body. These mechanisms include physical barriers such as skin, chemicals in the blood, and immune system cells that attack foreign cells in the body. The innate immune response is activated by chemical properties of the antigen.

Adaptive immunity Adaptive immunity refers to antigen-specific immune response. The adaptive immune response is more complex than the innate. The antigen first must be processed and recognized. Once an antigen has been recognized, the adaptive immune system creates an army of immune cells specifically designed to attack that antigen. Adaptive immunity also includes a "memory" that makes future responses against a specific antigen more efficient.

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The Immune System: Lines of Defence

First line of defence

The primary defence against infectious disease are the surface barriers that prevent pathogens from entering the body

These surface barriers include intact skin (protect external boundaries) and mucous membranes (protect internal boundaries)

Both the skin and mucous membranes release chemical secretions which restrict the growth of microbes on their surfaces

If pathogens cannot enter the host body, they cannot disrupt normal physiological functions and cause disease

Second line of defence

The second line of defence against infection are the non-specific cellular and molecular responses of the innate immune system

These defences do not differentiate between different types of pathogen and respond the same way upon every infection

Phagocytic leukocytes migrate to infection sites and engulf foreign bodies (dendritic cells then present antigens to lymphocytes)

Inflammatory responses increase capillary permeability at infected sites, recruiting leukocytes but leading to localised swelling

Antimicrobial proteins (such as cytokines and complement proteins) regulate immune activity within the body

Fever increases body temperatures to activate heat-shock proteins and suppress microbial growth and propagation

Third line of defence

The final line of defence against infection are the lymphocytes that produce antibodies to specific antigenic fragments

Each B cell produces a specific antibody, and the body has millions of different B cells capable of detecting distinct antigens

Helper T cells regulate B cell activation, ensuring that antibodies are only mass-produced at the appropriate times

Both B and T cells will differentiate to form memory cells after activation, conferring long-term immunity to a particular pathogen

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White Blood Cells LeucocytesNeutrophils

Most abundant type of white blood cell and the first responder to microbial infection They are unable to renew their lysosomes and die after having phagocytosed a few

pathogens (forms the majority of pus) Analogy:  Standard police officer – quick to respond to the situation, but lacks special

training or skills and so dies rapidly

Eosinophils

Prominent at the sites of allergic reactions and parasitic infections (rare in blood but common at mucous membranes)

Do not phagocytose pathogens but instead release chemical products which perforate cell membranes

Consequently, they function as the primary response to large multicellular parasites (e.g. helminth infections)

Analogy:  Fumigator – specialised to deal with pests / parasites (e.g. helminths) by releasing chemical products

Basophil

Basophils are chiefly responsible for initiating inflammatory responses by releasing the chemicals histamine and heparin

Functionally they are similar to mast cells, however they circulate in the bloodstream whereas mast cells are localised

Because they promote inflammation, they are common contributors to allergic responses Analogy:  Fireman – the leukocyte involved when a region is inflamed (‘in flames’)

Monocyte

Monocytes are the largest type of leukocyte and share phagocytosis duties with neutrophils They are slower to respond than neutrophils but are longer lasting, as they can renew their

lysosomes for continued digestion Monocytes will differentiate into two types of cells in response to pathogenic infection –

macrophages and dendritic cells Macrophages will remain in the tissue and phagocytose, whereas dendritic cells present

antigen fragments to lymphocytes Analogy:  Riot police (macrophage) – slower to respond than standard police but better

prepared and survives for longer Analogy:  Signalman (dendritic cell) – identifies the pathogen and sends signals to the

appropriate special forces (lymphocytes)

Lymphocyte

Lymphocytes are responsible for the production of antibodies which target specific antigens present on pathogens

They are more common in the lymphatic system than blood and are slowest to respond (requiring antigen presentation)

Lymphocytes include B cells (which become antibody-secreting plasma cells) and T cells (which mediate B cell activity)

Lymphocytes are also involved in the destruction of virus-infected body cells (via cytotoxic T cells and natural killer cells)

Analogy:  Special forces / superheroes – takes longest to mobilise but specially trained to target specific pathogens

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