Immunity in Humans, revision notes

Overview

The human body defends itself against disease-causing pathogens through three layers of defence that work together. This chapter opens with the physical and chemical barriers that stop most pathogens from entering, then explains how phagocytes and lymphocytes deal with anything that gets through, how antibodies recognise and disable specific antigens, and how active and passive immunity differ in how long their protection lasts.

You will also study how vaccination trains the immune system to respond quickly to a pathogen it has not truly met before, by introducing a weakened or dead antigen that triggers antibody and memory cell production without causing the disease itself. This links directly to why some illnesses can be prevented while others, once caught, still leave the body with lasting protection afterwards.

The final content standard covers health issues that arise when the immune system malfunctions in some way: allergy, where it overreacts to a harmless substance; autoimmune disease, where it attacks the body's own tissue; immunodeficiency, where a defence line such as the T-lymphocyte count is weakened, as in HIV infection; and the rejection of transplanted organs, where donor tissue is recognised as foreign.

Exam questions often ask you to compare terms that sound similar, such as antigen against antibody, or active against passive immunity, and to apply the three lines of defence to an unfamiliar scenario rather than simply listing them. Building a clear mental sequence, barrier, then phagocyte, then lymphocyte and antibody, makes both structured and essay-style questions easier to answer accurately.

The three lines of defence (Content Standard 11.1 Body Defence)

Content Standard 11.1 treats the body's defence as a layered system rather than a single wall. A pathogen that causes disease has to overcome three lines of defence, each one working in a different way, and the order in which they act is the backbone of almost every structured answer on this topic.

The first line of defence is a physical and chemical barrier that stops most pathogens from entering at all. The skin forms an unbroken physical covering; mucus in the nose, airways and gut traps pathogens and is then swept away or swallowed; tears and saliva contain the enzyme lysozyme that digests bacterial cell walls; and stomach acid kills most pathogens in swallowed food.

None of this barrier targets a particular pathogen, which is why it is described as non-specific.

The second line of defence is also non-specific and acts on anything that breaks through the barrier. Phagocytes, a type of white blood cell, move towards a pathogen, engulf it by phagocytosis and digest it with enzymes.

The third line of defence is specific: lymphocytes recognise a particular antigen on the pathogen and produce antibodies shaped to match only that antigen, and they also form memory cells that make any later response faster. Reading the table below from top to bottom gives the exact order a pathogen would meet these defences.

Phagocytes and lymphocytes: the cells that defend the body

Two groups of white blood cell carry out the second and third lines of defence, and a precise answer keeps their roles apart. Phagocytes act in the second line.

A phagocyte is drawn towards a pathogen by chemicals the pathogen releases, flows around it, takes it inside a vacuole and releases digestive enzymes that break it down. Because a phagocyte will engulf any foreign particle, its action is non-specific and needs no prior exposure.

Lymphocytes act in the third line and are specific. Each lymphocyte responds to one type of antigen, so the body holds an enormous variety of lymphocytes between them.

When a lymphocyte meets the antigen it matches, it multiplies rapidly and produces large numbers of antibodies against that antigen. Some of the new cells become memory cells that remain in the body long after the infection is cleared.

The link between the two groups is worth stating: antibodies made by lymphocytes often mark a pathogen so that phagocytes find and engulf it faster. The defence is therefore a cooperation between non-specific and specific cells rather than two separate systems working alone.

Antigens, antibodies and the actions of antibodies (Content Standard 11.2)

An antigen is a foreign marker, usually a protein, found on the surface of a pathogen, a toxin or a transplanted tissue. An antibody is a Y-shaped protein produced by lymphocytes with a binding site shaped to fit one specific antigen, much as a key fits one lock.

This specificity explains why an antibody made against one pathogen gives no protection against an unrelated one.

Antibodies do not digest pathogens themselves. They act in three ways.

By agglutination they clump many pathogens together so that a single phagocyte can engulf several at once. By neutralisation they bind to and inactivate the toxins that a pathogen releases, stopping the toxins from harming body cells.

By opsonisation they coat the pathogen so that phagocytes recognise and engulf it more quickly. The destruction that follows is still carried out by phagocytes, which is the point students most often miss.

A useful check in the exam is that an antigen is found on the invader while an antibody is made by the host to match it, and that antibodies label and disable a pathogen rather than eating it.

Types of immunity: active, passive, natural and artificial (Content Standard 11.3)

Immunity is classified in two overlapping ways, and the table below sets the four combinations side by side. Active immunity develops when the body's own lymphocytes make antibodies, either after a real infection (natural active) or after vaccination with a weakened or dead antigen (artificial active).

Because memory cells are formed, active immunity takes time to build but then lasts for years.

Passive immunity occurs when ready-made antibodies from outside enter the body: across the placenta and through breast milk from mother to baby (natural passive), or by an injection of antibodies after exposure to a dangerous pathogen (artificial passive). No memory cells are formed, so passive immunity acts at once but fades within weeks or months.

Most full-mark comparison answers turn on two contrasts: active immunity is slow to start but long-lasting because memory cells form, while passive immunity is immediate but short-lived because no memory cells form.

How vaccination gives long-term protection

A vaccine contains a weakened, dead or partial form of a pathogen, which acts as the antigen without causing the full disease. When the vaccine is given, lymphocytes recognise the antigen and respond as they would to a real infection: they multiply, produce antibodies, and form memory cells specific to that antigen.

The memory cells are the reason a vaccine protects for a long time. If the real pathogen enters the body later, the memory cells recognise its antigen at once and produce antibodies far faster and in much larger amounts than during the first exposure, so the pathogen is destroyed before it can cause illness.

This faster, stronger response is called the secondary response.

Booster doses are sometimes needed because the number of memory cells and the level of antibodies from a single dose can fall over time. A strong exam answer states that a vaccine supplies an antigen, not antibodies, and that the long-term protection comes from memory cells producing a rapid secondary response.

Health issues related to immunity (Content Standard 11.4)

The final content standard covers four situations in which the immune system does not behave normally, and each arises from a different fault. In an allergy the immune system overreacts to a normally harmless substance, called an allergen, such as pollen, dust mites or certain foods, releasing chemicals that cause sneezing, a rash or swelling; a severe reaction known as anaphylaxis needs urgent treatment.

In an autoimmune disease the immune system fails to tell the body's own cells from foreign antigens and attacks healthy tissue, so the organ that is targeted, such as the joints or the pancreas, decides the symptoms.

Immunodeficiency is a weakened or missing part of the defence system. HIV is a well-known cause: the virus infects and destroys T-lymphocytes, so as their number falls the body loses the ability to coordinate a response and infections it would normally clear become serious, the stage called AIDS.

Immunodeficiency can also be present from birth as a genetic condition. Organ transplant rejection happens because a transplanted organ carries antigens different from the recipient's own, so the immune system may recognise the graft as foreign and attack it.

Each of these is examined by asking which part of the normal response has gone wrong. The table below links each health issue to the fault behind it, which is the structure most answers are built around.

Close tissue matching between donor and recipient, and immunosuppressant drugs that reduce the immune response, lower the risk of rejection but do not remove it, which is why an organ from a close relative is often a better match.

Key concepts to master

• Body defence, The first line of defence, skin, mucus in the airways, and stomach acid, forms a physical and chemical barrier that stops most pathogens from entering the body. The second line is non-specific: phagocytes engulf and digest any pathogen that slips past the barrier. The third line is specific: lymphocytes recognise particular antigens and produce matching antibodies.
• Antibodies, Antibodies are Y-shaped proteins made by lymphocytes. Each type has a binding site shaped to match one specific antigen, much like a key fits one lock, so the body builds up a different antibody for every pathogen antigen it has encountered before. This specificity is why an antibody made against one pathogen normally gives no protection against a different, unrelated pathogen.
• Actions of antibodies, Antibodies act in three ways: they agglutinate pathogens by clumping them together so phagocytes can engulf many at once, they neutralise toxins that pathogens release, and they coat pathogens so phagocytes recognise and destroy them faster. This combined action clears an infection while limiting the damage a pathogen can do first.
• Active immunity, Active immunity develops when the body's own lymphocytes produce antibodies, either after fighting a real infection or after vaccination with a weakened antigen. Because memory cells are formed in the process, this protection takes time to build up but then lasts for years, sometimes for life.
• Passive immunity, Passive immunity happens when ready-made antibodies from another source enter the body, such as those a baby receives from its mother through the placenta or breast milk, or antibodies given by injection after exposure to a dangerous pathogen. Because no memory cells are made, protection is immediate but fades within weeks or months.
• Vaccination, Vaccination introduces a weakened, dead or partial form of a pathogen, the antigen, into the body without causing the disease itself. Lymphocytes respond by producing antibodies and, importantly, memory cells, so a real infection later triggers a much faster and stronger antibody response than the first exposure did.
• Allergy, An allergy is an exaggerated immune response to a normally harmless substance called an allergen, such as pollen, dust mites or certain foods. The immune system treats the allergen as dangerous and releases chemicals that cause symptoms such as sneezing, rashes or swelling; a severe reaction, anaphylaxis, needs urgent treatment.
• Autoimmune disease, An autoimmune disease occurs when the immune system fails to distinguish the body's own cells from foreign antigens and attacks healthy tissue as if it were a pathogen. The tissue that is targeted determines the symptoms, for example joint damage when the attack is directed at the lining of the joints. Common examples studied at this level include the immune system attacking the pancreas or the joints, which is why such conditions usually need long-term medical management rather than a single cure.
• Immunodeficiency, Immunodeficiency is a weakened or missing part of the immune system, leaving a person less able to fight infections that a healthy body would usually clear quickly. HIV is a well-known cause: the virus infects and destroys T-lymphocytes, so the immune system gradually loses its ability to coordinate a defence. Immunodeficiency can also be present from birth as a genetic condition, not only acquired later through infection.
• Organ transplant rejection, A transplanted organ carries antigens that differ from the recipient's own, so the recipient's immune system may recognise the graft as foreign and attack it. The risk of this rejection is reduced, though not removed, by matching donor and recipient tissue types closely and by giving immunosuppressant drugs. This is also why an organ from a close relative, who is more likely to share similar tissue antigens, is often a better match than one from an unrelated donor.

Quick recall checklist

1. Can you define and explain Body defence?
2. Can you define and explain Antibodies?
3. Can you define and explain Actions of antibodies?
4. Can you define and explain Active immunity?
5. Can you define and explain Passive immunity?
6. Can you define and explain Vaccination?
7. Can you define and explain Allergy?
8. Can you define and explain Autoimmune disease?
9. Can you define and explain Immunodeficiency?
10. Can you define and explain Organ transplant rejection?

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Source:SRC-DSKP-EN