Immunity

An antigen is a molecule, usually a protein or glycoprotein on a cell surface, that is recognised by the immune system as foreign (non-self) and that triggers an immune response, including the production of antibodies.

Your own cells carry 'self' antigens that the immune system has learnt to tolerate, so they are not recognised as foreign and do not stimulate a response.

The phagocyte is attracted to the pathogen by chemicals it releases, and receptors on the phagocyte's surface bind to antigens on the pathogen.

The phagocyte engulfs the pathogen by endocytosis, enclosing it in a vesicle called a phagosome (phagocytic vacuole).

Lysosomes inside the phagocyte fuse with the phagosome and release hydrolytic enzymes, such as lysozyme, into it.

These enzymes digest and break down the pathogen, and the soluble products are absorbed by the phagocyte.

In active immunity the person's own immune system is stimulated by an antigen to make antibodies and memory cells, so protection is slow to develop but long-lasting. An example is the immunity gained after catching an infection, or after vaccination.

In passive immunity the person receives ready-made antibodies from another source rather than making their own, so no memory cells are formed; protection is immediate but short-lived because the antibodies are eventually broken down. An example is the antibodies a baby receives from its mother across the placenta or in breast milk (or an injection of anti-venom).

Each B lymphocyte has a specific antibody on its surface. When a pathogen enters the body, only the B cell whose antibody is complementary in shape to the antigen is selected — this is clonal selection.

This selected B cell is stimulated to divide, partly by cytokines released from activated T helper cells.

The B cell divides repeatedly by mitosis to produce a clone of identical cells — a process called clonal expansion.

Most of these cells differentiate into plasma cells, which secrete large quantities of the specific antibody into the blood and lymph, where the antibodies bind to and help destroy the pathogen.

The remaining cells become B memory cells, which persist in the blood for a long time and divide rapidly into plasma cells if the same antigen is encountered again, giving a faster secondary response.

Antibodies work by binding specifically to antigens on a pathogen's surface, forming antigen–antibody complexes, and then disabling the pathogen so that other parts of the immune system can destroy it.

In agglutination, each antibody has two antigen-binding sites, so a single antibody can bind antigens on two different pathogens at the same time. This clumps many pathogens together. The clumps are too large to enter host cells and are easier for phagocytes to engulf, so one phagocyte can clear many pathogens at once.

In neutralisation, antibodies bind to toxins released by the pathogen, or to the surface molecules a pathogen uses to attach to and enter host cells. This blocks the toxin or prevents the pathogen entering cells, so it can no longer cause harm.

In opsonisation, antibodies coat the surface of the pathogen. Their constant regions act as markers that phagocytes recognise and bind to, stimulating phagocytosis. In each case the antibody marks or immobilises the pathogen, and phagocytes then engulf and digest it.

A vaccine contains antigens from a pathogen — for example a dead or inactivated pathogen, a weakened (attenuated) pathogen, or isolated antigens. When the vaccine is given, these antigens are recognised as foreign and trigger a primary immune response without causing the disease.

In this primary response, the specific B cell undergoes clonal selection and clonal expansion, producing plasma cells that secrete antibodies and, importantly, B memory cells and T memory cells that remain in the blood.

If the person is later infected by the real pathogen, the memory cells recognise its antigens immediately and divide rapidly into plasma cells. This produces a secondary response that is faster and much larger than the primary response, destroying the pathogen before symptoms develop, so the person is immune.

Herd immunity occurs when a high enough proportion of the population is vaccinated and therefore immune. Because so few people can catch and pass on the pathogen, transmission chains are broken and the pathogen cannot spread easily. This protects even unvaccinated individuals, such as newborn babies and people who cannot be vaccinated for medical reasons.

Monoclonal antibodies are identical antibodies, all specific to a single antigen, produced from one clone of cells.

First, an animal such as a mouse is injected with the chosen antigen. This stimulates its B lymphocytes (plasma cells) to make the specific antibody, but these plasma cells cannot divide indefinitely in culture.

The plasma cells are therefore fused with tumour (myeloma) cells, which divide indefinitely. The fused cells are called hybridoma cells, and they both divide rapidly and secrete the specific antibody.

The hybridoma cells are cultured, and the monoclonal antibody they produce is collected and purified.

One medical use is targeted cancer treatment: a cytotoxic drug is attached to the monoclonal antibody, which carries it directly to cancer cells that have the target antigen on their surface, killing them while sparing most healthy cells. (Monoclonal antibodies are also used in pregnancy tests and ELISA diagnosis.)

In an ELISA test, the target — here the virus antigen — is immobilised in a well, and the patient's blood is added so that any specific antibodies bind to it. A second antibody with an enzyme attached is then added; it binds only to the bound antibody/antigen complex. The well is washed to remove any unbound enzyme-linked antibody.

A substrate is then added. If the enzyme-linked antibody is present, the enzyme converts the substrate into a coloured product. The presence of colour therefore shows that the target is present, and the intensity of the colour — the absorbance — is proportional to the amount present. A washing step is essential so that only specifically bound antibody, not unbound antibody, produces colour.

Using the data and the threshold of 0.30: patients B (0.92) and C (0.35) have absorbance readings above 0.30, so they are infected. Patient B has by far the highest reading (0.92), indicating the highest concentration of antibody. Patients A (0.08) and D (0.11) are well below the threshold, so they are not infected.

A reliable test would also include a positive control (a known infected sample) and a negative control (a known uninfected sample) to confirm the colour change is due to the specific reaction and not a fault in the test.

The data show that the second (secondary) response is both faster and much greater than the first (primary) response. After the first dose the antibody concentration rose slowly, reaching its peak of only 12 au at day 16. After the booster it rose far more quickly, reaching a much higher peak of 95 au by day 5. The secondary response therefore reached its peak about 11 days sooner and produced roughly eight times as much antibody (95 ÷ 12 ≈ 7.9).

The primary response is slow and small because the specific B lymphocyte first has to be selected from the population (clonal selection) and then undergo clonal expansion by mitosis before plasma cells are present in large numbers to secrete antibody. This produces a delay, or lag, before antibody levels rise.

During the primary response, B memory cells are also produced, and these persist in the blood long after the antibody concentration has fallen.

When the booster delivers the same antigen, these memory cells recognise it immediately and divide very rapidly into many plasma cells. Because plasma cells appear much sooner and in greater numbers, antibody is produced faster and reaches a far higher concentration. This larger, faster secondary response is the basis of long-term immunity and is why booster doses are given.

Monoclonal antibodies are identical antibodies all specific to one antigen. Their greatest benefit is their specificity. Because each antibody binds to only one type of antigen, it can be used to target a particular kind of cell. In cancer treatment, a cytotoxic drug is attached to a monoclonal antibody that binds to an antigen found on the surface of the tumour cells. The antibody carries the drug directly to the cancer cells, so the drug acts mainly on the tumour and largely spares healthy cells. This reduces the harmful side-effects associated with conventional chemotherapy, which damages healthy dividing cells throughout the body. Monoclonal antibodies are also extremely useful in diagnosis — for example in pregnancy tests, ELISA tests for infections, and locating blood clots or tumours using labelled antibodies — because they detect one specific molecule reliably.

There are, however, risks and drawbacks. Monoclonal antibodies are difficult and expensive to produce, which limits how widely they can be used and raises questions about fair access. Antibodies produced in mice can be recognised as foreign by the human immune system, causing an immune reaction or reducing their effectiveness, although 'humanised' antibodies reduce this problem. Some patients have suffered serious side-effects in trials, so testing must be careful and thorough. There are also ethical concerns about the use of animals (mice and tumour cells) in their production.

Weighing these factors, the benefits are greatest where a highly specific, targeted treatment or test is needed and few alternatives exist — for example treating a specific cancer or rapidly diagnosing an infection. The high cost and the small risk of immune reactions are outweighed in these cases by the gain in effectiveness and the reduction in side-effects compared with non-targeted treatments.

In conclusion, monoclonal antibodies are a valuable medical tool whose benefits — high specificity, targeted treatment and reliable diagnosis — generally outweigh the risks for the conditions they are designed to treat, provided they are properly tested and patients are monitored. Because of their cost and the possibility of immune reactions, however, they should be used selectively rather than as a routine first-line treatment for every condition.