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.

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.

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.

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.

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.