The body's defences

The diagram shows the body's three lines of defence. Write the name of the lines labelled A, B, and C, ordering them from outermost to innermost, and indicate which of them is specific to each infectious agent.

A small scratch heals on its own, without treatment. Describe the chain of defences that acts from the moment the skin is broken until the wound closes, indicating which line of defence intervenes at each moment.

Name four body barriers that hinder the entry of microorganisms (skin, mucous membranes, tears, gastric acid...) and explain how each of them protects the body.

Explain what role mucus and the mucous membranes of the airways play as a defensive barrier against microorganisms.

An extensive burn is a serious emergency even if there is no infection at first. Explain, relating it to the lines of defence, why burned skin puts life in danger and what risk appears in the following days.

Explain what phagocytes are, which group of blood components they belong to, and why they are said to "eat" microbes.

Explain what inflammation is and what its four characteristic signs are in the affected area.

When you get a splinter, the area becomes red, warm, and swollen. Explain what is happening, what this response is called, which blood cells travel to the site, and why this reaction, although annoying, is beneficial.

Fever is a controlled increase in body temperature during many infections. Reason why it can be considered a defence mechanism and not just a symptom, and explain in which situations it should indeed be treated.

While walking through a park in spring, Cian starts sneezing non-stop, his eyes water, and the skin on his neck turns red. A friend tells him: "You've caught a very aggressive bacterium; take an antibiotic quickly to cure yourself". a) Refute the two scientific errors in his friend's statement, explaining what is actually happening in Cian's immune system in response to pollen. b) The doctor prescribes an "antihistamine" drug. Predict and explain what effect this medication will have on his blood vessels and mucous membranes based on the action that histamine exerts during an allergic reaction.

A teenager has a body temperature of 37.9 °C and a sore throat with mild inflammation. His mother wants to immediately give him high doses of antipyretics and anti-inflammatory drugs so that his temperature drops to 36.5 °C right away. a) Based on the defensive role of moderate fever and inflammation (arrival of phagocytes, increased cellular metabolism, and slowing down of microbial reproduction), justify why completely suppressing these mild symptoms could prolong the infection. b) Predict and explain what life-threatening danger would occur if the fever continuously exceeded 41.5 °C, explaining the effect of extreme heat on the proteins and enzymes of our cells.

State whether these statements are true or false and justify your answer: a) fever must always be brought down immediately with medication; b) antibodies are proteins made by the body itself; c) the first time a microbe enters, the specific response takes longer than the non-specific one.

Explain what an antibody is, which cells produce it, and why each antibody only acts against a specific infectious agent.

Explain what lymphocytes are and what two major tasks they carry out within the specific response.

Compare the non-specific and specific immune responses: speed of action, cells involved, memory against the same agent, and an example of how each acts during the same infection.

People whose spleen has been removed are advised to get more vaccines and to watch out for any infection. Explain what relationship the spleen has with white blood cells and why its absence weakens the defences without cancelling them out.

During flu, the glands in the neck swell. Explain what the lymph nodes are, what happens inside them during an infection, and why their swelling indicates that the defences are at work.

Against the same bacterium, the phagocytes act first and the lymphocytes afterwards. Compare both responses: order of arrival, way of recognising the invader, and what remains in the body when each one finishes its work.

A patient undergoing chemotherapy is diagnosed with pneumonia caused by Pneumocystis jirovecii, a harmless fungus that coexists in the respiratory tract of healthy people without causing any harm. Analyse this clinical case: a) Justify why this infection is described as "opportunistic", supporting your answer with the state of the patient's defences following treatment. b) If a healthy family member lives in close contact with him, predict whether they will fall ill from this same fungus and explain your reasoning. c) Design two specific preventive measures that the hospital or family environment should implement to protect him from other common pathogens.

In a study, the amount of antibodies in a person's blood against a pathogenic bacterium is measured at two different times: after the first contact (day 1), antibodies take 10 days to appear and reach a maximum level of 15 units; after a second infection one year later (day 365), antibodies reach 250 units in just 2 days. a) Interpret these results, justifying the difference in speed and quantity based on the function of memory lymphocytes. b) Justify how vaccines take advantage of this same biological principle to protect us without having to suffer from the disease.

A person claims: "Getting the flu vaccine is useless; my aunt got vaccinated in November and in January she was in bed with the flu, feeling unwell and coughing." Refute their conclusion using your knowledge of the immune response: a) Justify why the flu virus can infect a vaccinated person if it has undergone minor mutations in its surface antigens and why this requires changing the vaccine every year. b) Predict and justify what the course of infection would have been like in their aunt had she not been vaccinated, explaining the role of the antibodies and lymphocytes generated by the previous vaccine.

Helper T lymphocytes act as central coordinators of the immune system, as they provide the chemical signal to activate both B lymphocytes (which make antibodies) and macrophages and cytotoxic lymphocytes. The HIV virus specifically infects and destroys these cells. a) Predict what chain of consequences the defense system suffers by losing these specific lymphocytes and deduce why patients with advanced AIDS do not die from the virus itself, but from other infections. b) If a combination of antiretroviral drugs halts the multiplication of the virus until it becomes undetectable, predict what will happen to the lymphocyte count and the patient's health.

In type 1 diabetes, lymphocytes selectively destroy the beta cells of the pancreas (responsible for producing insulin) by mistaking their membrane proteins for those of a dangerous pathogen. a) Analyze this process using precisely the terms "antigen", "autoantibody", and "self-tolerance", explaining how this attack differs from one against an invading bacterium. b) A student suggests: "Since it is a cellular attack, we could definitively cure diabetes by administering antibiotics or destroying the patient's entire immune system." Reasonably refute both proposals.

Aoife has had a kidney transplant and must take immunosuppressive drugs daily. After a few months, her doctors adjust the dose with extreme caution: if they increase the amount too much, Aoife frequently contracts bacterial and fungal infections; if they lower the dose, the transplanted kidney begins to lose its filtering function. a) Justify the biological dilemma that explains this delicate balance, identifying which components of the immune system are involved in each of the two risks. b) If Aoife has to go to a health center during a flu outbreak, predict what specific risks she runs and justify two essential self-protection measures she must adopt.