Immunity in Humans, practice questions

C, Stomach acid is a chemical barrier of the first line; phagocytes and lymphocytes belong to the second and third lines.

B, Phagocytes carry out phagocytosis, engulfing and digesting any pathogen that gets past the barrier; antibody production is the job of lymphocytes.

B, An antibody is a Y-shaped protein made by lymphocytes with a binding site shaped to fit one specific antigen.

B, Agglutination is the clumping of pathogens; neutralisation inactivates toxins, and phagocytosis is the engulfing carried out by phagocytes.

A, Antibodies passed from mother to baby are received ready-made, making this natural passive immunity; the others involve the body making its own antibodies.

C, Active immunity involves the body making its own antibodies and forming memory cells, so it develops slowly but lasts for years.

B, The memory cells formed in response to the vaccine allow a rapid, large antibody response if the real pathogen is met later.

B, In an allergy the immune system treats a harmless substance such as pollen as a threat and overreacts; the other options describe autoimmune disease, immunodeficiency and transplant rejection.

B, HIV infects and destroys T-lymphocytes, so the body loses the ability to coordinate an effective specific immune response.

B, The donor organ carries antigens different from the recipient's own, so the immune system may recognise the graft as foreign and attack it; tissue matching and immunosuppressant drugs reduce this risk.

• First line: a physical and chemical barrier (skin, mucus, stomach acid, lysozyme) that normally stops pathogens entering.
• Second line: phagocytes engulf the pathogen by phagocytosis and digest it with enzymes; both lines are non-specific.
• Third line: lymphocytes recognise the specific antigen on the pathogen.
• The lymphocytes multiply and produce antibodies that match only that antigen, and form memory cells.
• In a second infection the memory cells recognise the antigen at once.
• They produce antibodies much faster and in larger amounts (a secondary response), so the pathogen is destroyed before it causes illness.

• From the mother: natural passive immunity.
• It is short-lived because the baby receives ready-made antibodies and does not form memory cells, so protection fades as the antibodies break down.
• From the vaccine: artificial active immunity.
• The vaccine antigen makes the baby's own lymphocytes produce antibodies and form memory cells.
• The memory cells remain in the body and give a fast, large secondary response if the real pathogen is met.
• This lasting memory response is why the vaccine protects for far longer than the borrowed maternal antibodies.

• Hay fever (allergy): the immune system overreacts to a harmless allergen such as pollen, releasing chemicals that cause sneezing and other symptoms.
• Autoimmune disease: the immune system fails to tell the body's own cells from foreign antigens and attacks healthy tissue.
• Advanced HIV infection: HIV destroys T-lymphocytes, so the specific immune response can no longer be coordinated and infections become serious (AIDS).
• The autoimmune attack is directed at the body's own tissue rather than an outside pathogen.
• There is no single pathogen to remove, so treatment reduces the misdirected immune activity instead of eliminating a cause.
• The condition is therefore controlled long-term to limit tissue damage rather than cured outright.

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 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.

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 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 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 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.

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.

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 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.

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.