Immune response

Where it happens

The immune response takes place in the blood and lymphatic system, wherever lymphocytes, a type of white blood cell, encounter a pathogen's antigens. Unlike phagocytosis, it is specific: each type of lymphocyte responds to one particular antigen.

Lymphocytes are made in the bone marrow and gather in the lymph nodes, spleen and tonsils. Lymph draining from an infected tissue carries antigens into the nearest lymph node, which is where most lymphocytes first meet them; this is why lymph nodes in the neck swell during a throat infection.

Once activated, the lymphocytes and the antibodies they release travel in the blood plasma and tissue fluid to reach the pathogen wherever it is.

Inputs and outputs

• Input: antigens, the foreign molecules on the surface of a pathogen, or on a toxin or a transplanted cell.
• Input: lymphocytes carrying a receptor whose shape matches that antigen.
• Input: amino acids and ATP, used by the activated lymphocytes to divide and to build antibody proteins.
• Output: antibodies, proteins with a binding site complementary to the antigen, released into the blood plasma.
• Output: destroyed pathogens, either clumped together by antibodies and engulfed by phagocytes, or broken down directly.
• Output: memory cells, which stay in the blood and lymph for years and give immunity to that pathogen.

The steps

1. A pathogen enters the body carrying antigens, molecules on its surface that the body recognises as foreign.
2. Phagocytes engulf some of the pathogens and display their antigens on their own surface, alerting lymphocytes.
3. Lymphocytes with a matching receptor recognise the antigen and become activated.
4. Activated lymphocytes multiply rapidly by mitosis, producing a clone of identical cells.
5. Some of these cells produce antibodies, which bind specifically to the antigen and help destroy the pathogen or mark it for destruction.
6. Antibodies clump the pathogens together, neutralise their toxins, or make them easier for phagocytes to engulf.
7. Some of these cells remain in the body as memory cells, ready to respond faster if the same antigen appears again.

Why it matters and how it is controlled

The immune response allows the body to target a particular pathogen precisely, using antibodies shaped to match its antigens. Because memory cells remain after an infection, a second exposure to the same pathogen triggers antibody production more quickly and in larger amounts, often before symptoms of illness develop.

The difference between the first and second exposure is called the primary and secondary response. In the primary response there is a delay of days while the few matching lymphocytes are found and multiplied, and the antibody level rises slowly and stays low.

In the secondary response, the large population of memory cells reacts at once, so the antibody level rises within a day or two, peaks much higher and lasts longer. A graph of antibody concentration against time for the two exposures is a standard exam figure.

Vaccination uses this control deliberately. A vaccine contains dead or weakened pathogens, or their antigens, which trigger a primary response and leave memory cells without causing the disease.

When the real pathogen arrives later, the body mounts a secondary response straight away.

How it is examined

You may be asked to describe the stages from antigen recognition to antibody production, to explain the role of memory cells, or to compare the specific immune response with the non-specific defence carried out by phagocytosis.

The antibody-level graph appears in structured items: you label the primary and secondary response, explain why the second peak is higher and faster, and link it to memory cells. Vaccination questions ask you to explain how a vaccine gives immunity in terms of the same steps, or to compare active immunity from vaccination with passive immunity from antibodies in breast milk or an injection.

Essay items on the body's defences expect the immune response to be placed alongside the skin, mucus, blood clotting and phagocytosis as the specific third line of defence.

Common misconceptions

Worked exam-style question

Question. A child was infected by a virus and recovered after two weeks. Eight months later the same virus infected the child again, but no symptoms appeared.

The graph below records the concentration of antibody in the child's blood during both infections, with the second peak far higher and reached within days. (a) Name the type of white blood cell that produces the antibody.

(b) Explain why the antibody concentration rose slowly during the first infection. (c) Explain why the second infection produced a faster and larger rise, and why no symptoms appeared.

Model answer. (a) Lymphocytes. (b) During the first infection, only a small number of lymphocytes had a receptor matching the viral antigen.

These had to be activated and then multiply by mitosis before enough cells existed to make antibody, so there was a delay and the antibody concentration rose slowly; this is the primary response. (c) After the first infection, memory cells specific to the virus remained in the blood and lymph.

On the second exposure they recognised the antigen immediately and divided rapidly into antibody-producing cells, so antibody was made sooner and in larger amounts; this is the secondary response. The virus was destroyed before it could multiply enough to cause symptoms, which is what is meant by immunity.

Source:SRC-DSKP-EN

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