Q.Name any two physiological barriers that provide innate immunity.

Step 1 — What a vaccine actually delivers.

In immunisation, a preparation of antigenic proteins of a pathogen, or a weakened/inactivated pathogen (the vaccine), is introduced into the body. It is not itself protective — it cannot fight anything. Its job is to be seen.

Step 2 — What the body does with it.

The antigens generate a primary response: antibodies are produced, and crucially, memory B-cells and memory T-cells are generated that recognise that specific pathogen.

Step 3 — What happens on real infection.

When the vaccinated person is later actually infected, the existing memory cells recognise the pathogen at once and mount a secondary (anamnestic) response — faster, of much higher magnitude, and dominated by high-affinity antibodies. The pathogen is neutralised before disease develops.

This two-step logic is the principle of vaccination, and its load-bearing element is memory.

Step 4 — Why the other properties, though real, are not the principle. …

Step 1 — Read the stem as a definition.

"Chemical substances produced by some microbes which can kill or retard the growth of other microbes" — the two clauses are (i) microbial origin, and (ii) antimicrobial action. Only one class of molecule is defined by exactly those two clauses.

Step 2 — The word itself.

Anti- = against, bio = life. The name was coined because these substances act against (kill or retard) other living microbes. Alexander Fleming's chance observation of Penicillium notatum inhibiting Staphylococci on a culture plate led to Penicillin, whose full potential as an antibiotic was established by Ernst Chain and Howard Florey; the three shared the Nobel Prize in 1945. Penicillin was used extensively to treat wounded American soldiers in World War II.

Step 3 — Why each distractor fails the definition.

• (A) Statins — produced by the yeast Monascus purpureus, used as blood-cholesterol-lowering agents. They act by competitively inhibiting the enzyme responsible for cholesterol synthesis in humans — not by killing microbes. ✗
• (B) Streptokinase — produced by the bacterium Streptococcus and modified by genetic engineering; used as a "clot buster" to remove clots from the blood vessels of myocardial-infarction patients. Again, its target is a human clot, not a microbe. ✗ …

Step 1 — The four species that infect humans.

Step 2 — Why falciparum is the fatal one. …

Step 1 — What LAB does in milk.

Lactic Acid Bacteria (LAB), typically Lactobacillus, are added to warm milk as a starter. They ferment the milk sugar lactose into lactic acid. The acid lowers the pH, which coagulates the milk protein casein and gives curd its texture and sour taste.

Step 2 — The nutritional bonus (the point of the question).

Beyond coagulation, LAB carry out their own metabolism inside the milk, and a by-product of that metabolism is the synthesis of vitamin B$_{12}$ (cobalamin). Curd therefore contains more vitamin B$_{12}$ than the milk it was made from — this is the textbook reason given for curd being nutritionally superior to milk.

Step 3 — Why the other options fail.

• Vitamin C — a plant/citrus-associated vitamin; not a fermentation product of LAB in milk. …

The key here is understanding how the immune response is generated. When a person is infected with a deadly microbe, time is critical. The body's own immune system (active immunity) takes days to weeks to mount a full response — it needs to recognise the pathogen, activate B-cells, and then produce antibodies. That delay can be fatal.

So instead, we give preformed antibodies — antibodies that were already made in another organism (like a horse or another human) and are injected directly into the patient. This bypasses the entire production process. The immune response is immediate because the antibodies are ready to neutralise the microbe right away.

This is the defining feature of passive immunisation: the host receives ready-made immune components, rather than producing them itself. The protection is fast but short-lived, because the injected antibodies are eventually broken down and no memory cells are formed.

Now let's look at each option:

1. Active immunity — This is when the body's own immune system is activated and produces antibodies and memory cells. It can happen naturally (after an infection) or artificially (via vaccination). It takes time to develop but provides long-lasting protection. That's not what's happening here — the antibodies are given, not made by the patient.

2. Cell-mediated immunity — This involves T-cells directly attacking infected cells, not antibodies. The question specifically mentions preformed antibodies, which are part of the humoral (antibody-mediated) response, not cell-mediated. So this is incorrect. …

The key here is not about the chemical nature of cellulose itself, but about who has the right microbes. Both ruminants and humans eat plant matter containing cellulose, but only ruminants can extract nutrition from it. Why? Because they host a specialized community of microorganisms — including methanogens — in their digestive system that produce the enzyme cellulase. Humans don't have these microbes, so cellulose passes through undigested.

Let’s walk through the options carefully.

1. Option (A): Methanogens are present in human gut.

   This is false. Methanogens are archaea that produce methane and are found in the rumen of cattle, not in the human gut in any significant, functional way. Humans do have some gut microbes, but not methanogens that digest cellulose.

2. Option (B): Cellulose is a complex sugar.

   True, but irrelevant to the difference. Cellulose is indeed a polysaccharide (a complex carbohydrate), but that doesn’t explain why ruminants can digest it and humans cannot. Both face the same chemical — the difference lies in the digestive machinery.

3. Option (C): Cellulose reduces the bulk of food.

   This is incorrect. Cellulose actually adds bulk (roughage) to food; it doesn’t reduce it. In humans, it acts as dietary fibre, aiding bowel movement but providing no calories.

4. Option (D): Methanogens are absent in human gut. …

1. Pneumonia — bacterial disease (Statement A)

   Pneumonia is most commonly caused by the bacterium Streptococcus pneumoniae (though viral and fungal pneumonias also exist). In the context of this question, the statement is taken as correct because the classic, textbook form of pneumonia is bacterial. So (A) is true.

2. Ringworm — fungal disease (Statement B)

   Ringworm is a superficial fungal infection of the skin, caused by dermatophytes like Trichophyton, Microsporum, and Epidermophyton. It is not a worm at all — the name comes from the ring‑shaped rash. Statement (B) is correct.

3. HIV transmission — mosquito bite (Statement C)

   HIV (Human Immunodeficiency Virus) is transmitted through direct contact with infected blood, semen, vaginal fluids, or breast milk. Mosquitoes do not transmit HIV. Even if a mosquito bites an infected person, the virus does not replicate inside the insect and is not injected into the next person. The mosquito’s proboscis is a one‑way valve — it only injects saliva (which does not contain HIV), not blood from a previous meal. This is a classic misconception. Statement (C) is false. …