Physical and Chemical Responses in Animals

Analyse is this dot point's own verb — analyse responses to the presence of pathogens by assessing the physical and chemical changes. The marks are for how the changes relate to one another, not for listing four signs.

Plan: identify the components, state what links them, relate them across the response, then draw the implication and the limits.

Inflammation presents as four physical changes — redness, heat, swelling and pain — accompanied by chemical changes: the release of histamine, prostaglandins, cytokines and chemokines from damaged tissue and from mast cells.

The four physical signs are not four separate events but consequences of one chemical change. Histamine causes vasodilation and increases capillary permeability, and everything visible follows: increased blood flow produces the redness and the heat, and fluid and protein leaking into the tissue produce the swelling, which together with prostaglandins sensitising nerve endings produces the pain.

Each change serves the same purpose, which is to bring defence to the site. Increased blood flow delivers more phagocytes and plasma proteins; increased permeability lets those cells and proteins leave the vessel and enter the tissue; chemokines form a concentration gradient that guides neutrophils to the exact site; and cytokines signal more widely, raising body temperature and mobilising cells from bone marrow. Even the pain contributes, since it enforces rest of the injured part and so limits further damage and spread.

The analysis has limits, and they are where inflammation becomes the problem. The same mediators released systemically rather than locally cause widespread vasodilation and a catastrophic fall in blood pressure, which is septic shock. Sustained inflammation without resolution damages the tissue it is defending, as in rheumatoid arthritis. And the response is not pathogen-specific — it is triggered by any tissue damage, including sterile injury — so it cannot distinguish an infection from a bruise.

It follows that the symptoms of inflammation are the defence rather than the disease, so suppressing them trades comfort against function — an anti-inflammatory reduces prostaglandin production and therefore pain and swelling, while also reducing the delivery of defensive cells. The response is best understood as a general-purpose alarm that buys time for the specific immune response to develop, which is why its speed matters more than its precision.

The dot point pairs analyse with assessing the changes that occur, so this card takes the second verb. Assess means judging how effective these defences are against criteria, not describing them.

Plan: state the criteria, assess the first line, assess the second, then reach a verdict and qualify it.

The two lines will be assessed on how many pathogens they exclude or clear without further help, on how quickly they act, and on what they cost the host when they operate.

The first line — intact skin, mucus and cilia, stomach acid, lysozyme in tears and saliva, and the normal microbiota competing for space and nutrients — is highly effective and almost entirely invisible. It excludes the overwhelming majority of microorganisms encountered daily, requires no activation and imposes no cost when working. Its effectiveness is best judged by what happens when it fails: a burn destroying skin, or antibiotics disrupting gut microbiota, produces infection by organisms that were being excluded all along.

The second line — inflammation, phagocytosis, fever, the complement system and natural killer cells — is fast and broad rather than thorough. It begins within minutes and acts against any pathogen without prior exposure, which is its central advantage, and it clears many infections outright. But it is non-specific, so it cannot target a particular pathogen efficiently, it has no memory, so a second exposure is handled no better than the first, and some pathogens evade it directly — capsules resist phagocytosis and intracellular pathogens hide from it entirely.

Both lines impose costs when they operate. Inflammation damages host tissue, fever raises metabolic demand and is dangerous above about forty degrees, and an excessive systemic response can kill through septic shock. So the second line's effectiveness is bounded not only by what pathogens escape it but by how much of it the host can tolerate.

The two lines together are highly effective for the role they occupy and insufficient on their own for the infections that matter clinically. The first line is the most effective defence the body has, judged by the number of encounters it resolves; the second line is effective as a rapid, broad holding action but cannot eliminate a pathogen adapted to evade it. Their real function is to exclude the many and delay the few for the several days the adaptive response requires — so they should be judged on speed and coverage rather than on completeness.

The qualification is that the two systems are not independent, so assessing them separately understates both. Phagocytes of the second line present antigen to lymphocytes and so initiate the adaptive response, and antibodies produced adaptively then make phagocytosis far more efficient — the innate response is therefore both the delay that permits adaptive immunity and a system whose own effectiveness improves once adaptive immunity arrives.