Phagocytosis
- A phagocyte (e.g. macrophage) is a type of white blood cell that carries out phagocytosis (engulfment of pathogens).
- They’re found in the blood and in tissues and are the first cells to respond to an immune system trigger inside the body.
- A phagocytes recognises the foreign antigens on a pathogen. The membrane of the phagocyte then projects out and the cytoplasm of the phagocyte moves round the pathogen (engulfing it) as it is enclosed in a phagocytic vacuole.
- The lysosomes (contain the enzymes lysozymes) fuse with the phagocytic vacuole and release enzymes into the vacuole. The enzymes breakdown the pathogen. The phagocyte then presents the pathogen’s antigens – it sticks the antigens on its surface to activate other immune system cells.

1) The bacteria will be attracted to the membrane of the neutrophil.
2) Phagocytosis. The neutrophil will engulf the bacteria.
3) Lysosomes (vesicles containing digestive enzymes) will fuse with the phagosome containing the bacteria.
4) Now the bacteria will be killed and digested by enzymes.
The lymphocytes will also kill bacteria. However, some bacteria may escape by having a protective cell wall or capsule.
Phagocytes activate t-cells
- A T-cell (also called a T-lymphocyte) is another type of white blood cell. It has
- receptor proteins on its surface that bind to complementary antigens presented to it by phagocytes.
- This activates the T-cell. Different types of T-cells respond in different ways. For example, helper T-cells (TH cells) release chemical signals that activate and stimulate TH cells also activate B-cells which secrete antibodies.

B-Cells
B-cells (also called B-lymphocytes) are also a type of white blood cell. They’re covered with antibodies – proteins that bind antigens to form an antigen-antibody complex. Each B-cell has a different shaped antibody on its membrane, so different ones bind to different shaped antigens.
- When the antibody on the surface of a B-cell meets a complementary shaped antigen, it binds to it. This, together with the substance released from helper T-cells, activates the B-cell. This process is called clonal selection. The activated B-cell divides into plasma cells.

Plasma cells
Plasma cells are identical to B-cells (they’re clones). They secrete loads of antibodies specific to the antigen. These are called monoclonal antibodies. They bind to the antigens on the surface of the pathogen to form lots of antigen-antibody complexes.
- An antibody has two binding sites, so can bind to two pathogens at the same time. This means that pathogens become clumped together – this is called agglutination.
- Phagocytes then bind to the antibodies and phagocytose many pathogens at once. This process leads to the destruction of pathogens carrying this this antigen in the body.

The Immune Response
In IBDP Biology, you have to understand the immune response.
Antigens are molecules (usually proteins) that can generate an immune response when detected by the body. They are usually found on the surface of cells and are used in the immune system to identify:
- pathogens (organisms that cause disease)
- abnormal body cell (e.g. cancerous or pathogen- infected cells, which have abnormal antigens on the surface)
- toxins
- cells from other individuals of the same species (e.g. organ transplants)
Cellular – the T-cells and other immune system cells that they interact with, e.g. phagocytes, from the cellular response.
Humoral – B-cells, clonal selection and the production of monoclonal antibodies form the humoral response.
- Both types of responses are needed to remove a pathogen from the body and the responses interact with each other, e.g. T-cells help to activate B-cells, and antibodies coat pathogens making it easier for phagocytes to engulf them.
The Primary Immune Response
- When an antigen enters the body for the
- first time it activates the immune system.
- The immune system takes little time to respond and is
- slow because activated B cells are differentiating into plasma cells (there aren’t many B-cells that can make the antibody needed to bind to it).
- Eventually the body will produce enough of the right antibody to overcome the infection. Meanwhile the infected person will show symptoms of the disease.
- After being expose to an antigen, both T- and B-cells produce memory cells. These memory cells remain in the body for a long time. Memory T-cells recognise the specific antigen a second time round. Memory B-cells record the specific antibodies needed to bind the antigen.
- The person is now immune – their immune system has the ability to respond quickly to a second infection.
The Secondary Immune Response
- If the same pathogen (antigen) enters the body again, the immune system will produce a quicker immune response and more antibodies are produced.
- Clonal selection happens faster. Memory B-cells are activated and divide into plasma cells that produce the right antibody to the antigen. Memory T-cells are activated and divide into the correct type of T-cells to kill the cell carrying the antigen.
- The secondary response often gets rid of the pathogen before you begin to show any symptoms (you are immune to the pathogen).

Vaccinations
While B-cells are dividing to build their numbers to deal with a pathogen (i.e. the primary response), you suffer from the disease. Vaccination can help avoid this.
- Vaccines contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing disease. This means you become immune without any of the symptoms.
- Vaccines protect individuals that have them and, because they reduce the occurrence of the disease, those not vaccinated are also less likely to catch the disease. This is called herd immunity.
- Vaccines always contain antigens – these may be isolated or attached to a dead or attenuated (weakened) pathogen.
- Vaccines may be injected or taken orally. The disadvantages of taking a vaccine orally is that it could be broken down by enzymes in the gut or the molecules of the vaccine may be too large to be absorbed by the blood.
- Sometimes booster vaccines are given later on (e.g. after several years) to make sure that memory cells are produced; which increase the antibody concentration.
Antigenic Variation
- Some pathogens can change their surface antigens. This antigen variability is called antigen variation – different antigens are formed due to changes in the genes of the pathogen.
- This means that when you’re infected for a second time, the memory cells produced from the first infection will not recognise the different antigens. So the immune system has to start again and carry out a primary response against these new antigens.
- This primary response takes time to get rid of the infection, which is why you get ill again.
- Antigenic variation also makes it difficult to develop vaccines against some pathogens for the same reason. Examples of pathogens that show antigenic variation include HIV and the influenza virus.
Why the Influenza (flu) Vaccine Changes Every Year?
- That’s because the antigens on their surface
- change regularly, forming new strands of the virus.
- Memory cells
- produced from vaccination with one strain of the flu will not recognise other strains with different antigens. The strains are immunologically distinct.
- Every year there are different strains of the influenza virus circulating in the population, so a different vaccine has to be made.
- New vaccines are developed and one is chosen every year that is most effective against the recently circulating influenza viruses.
- Governments and health authorities then implement a programme of vaccination using the most suitable method.
Active and Passive Immunity
Active Immunity
The body makes its own antibodies after being stimulated by the antigen, along with memory cells.
- Natural – this is when you become immune after catching the disease.
- Artificial – this is when you become immune after you’ve been given a vaccination containing a harmless dose of antigen.
Passive Immunity
Antibodies are given from a different organism; your immune system doesn’t produce antibodies of its own.
- Natural - This could be due to the transfer of antibodies from the mother to the baby across the placenta and in breast milk. No memory cells involved so it’s a short term solution.
- Artificial – this is when you become immune after being injected with antibodies from someone else. E.g. if you contract tetanus you can be injected with antibodies against the tetanus toxin, collected from blood donations.

Antibodies in medicine
Targeting drugs to a particular cell type – cancer cells
- Different cells in the body have different surface antigens.
- Cancer cells have antigens that called tumour markers that are not found on normal body cells.
- Monoclonal antibodies can be made that will bind to tumour markers.
- You can also attach anti-cancer drugs to the antibodies.
- When the antibodies come into contact with cancer cells they will bind to the tumour markers.
- This means the drug will only accumulate in the body where there are cancer cells.
- So, the side effects of an antibody-based drugs are lower than other drugs because they accumulate near specific cells.
Targeting a particular substance for medical diagnosis –Pregnancy testing
Pregnancy tests detect the hormone human chronic gonadotropin (hCG) that’s found in the urine of pregnant women:
- If there is hCG present the test strip turns blue because the immobilised antibody binds to any hCG – concentrating the hCG antibody-complex with the blue beads attached. If no hCG is present, the beads will pass through the test area without binding to anything and so it won’t go blue.

Indirect ELISA Test (enzyme-linked immunosorbent assay)
- HIV antigen is bound to the bottom of a well in a well plate.
- A sample of the patient’s blood plasma, which might contain several different antibodies, is added to the well. If there are any HIV-specific antibodies (i.e. antibodies against HIV) these will bind to the HIV antigen stuck to the bottom of the well. The well is then washed out to remove any unbound antibodies.
- A secondary antibody, that has a specific enzyme attached to it, is added to the well. This secondary antibody can bind to the HIV-specific antibody (which is also called the primary antibody). The well is washed out again to remove any unbound secondary antibody. If there’s no primary antibody in the sample, all the secondary antibody will be washed away.
- A solution is added to the well. This solution contains a substrate, which is able to react with the enzyme attached to the secondary antibody and produce a coloured product. If the solution changes colour, it indicates that the patient has HIV-specific antibodies in their blood and is infected with HIV.

Ethical Issues
- All vaccines are tested on animals before they are tested on humans – some people disagree with
- animal testing. Also, animal based substances may be used to produce a vaccine, which some people disagree with.
- It can be difficult to test vaccines on humans, e.g. volunteers may put themselves at unnecessary risks of contracting the disease because they think they’re fully protected (e.g. they might have unprotected sex because they had a new HIV vaccine and the vaccine might not work).
- Some people don’t want to take vaccines sue to the risk of side effects, but are still protected because of herd immunity – other people think this is unfair.
- If there was an epidemic of a new disease (e.g. a new influenza virus) there would be a rush to receive a vaccine and difficult decision would have to be made about who would be the first to receive it.
Animal right issues
– animals are used to produce the cells from which the monoclonal antibodies are produced. Some people disagree with the use of animals in this way.
Antibiotics and cures
Antibiotics can kill bacteria by interfering with their metabolic reactions. By targeting bacterial enzymes and ribosomes used in the reactions. These are different from human enzymes and ribosomes; so human cells don't get damaged.
- Viruses don't have their own enzymes and ribosomes – they use host’s cells to replicate. Antibiotics can‘t inhibit them because they don't target human processes.
- Most antiviral drugs designed to target few virus-specific enzymes that exist (enzymes that only the virus uses). For example, HIV uses reverse transcriptase to replicate. Human cells don’t use this enzyme, so drugs can be designed to inhibit it without affecting the host cell. These drugs are called reverse transcriptase inhibitors.
This is the end of the topic

Drafted by Eva (Biology)
Photo references:
- https://teachmephysiology.com/immune-system/innate-immune-system/phagocytosis/
- https://www.thermofisher.com/hk/en/home/life-science/cell-analysis/cell-analysis-learning-center/immunology-at-work/cytotoxic-t-cell-overview.html
- https://www.immunology.org/public-information/bitesized-immunology/immune-development/b-cell-activation-and-the-germinal-centre
- https://www.mrgscience.com/topic-111-antibody-production-and-vaccination.html
- https://microbiologynotes.com/differences-between-primary-and-secondary-immune-response/
- https://ib.bioninja.com.au/higher-level/topic-11-animal-physiology/111-antibody-production-and/types-of-immunity.html
- https://ib.bioninja.com.au/higher-level/topic-11-animal-physiology/111-antibody-production-and/monoclonal-antibodies.html
- https://tw.sinobiological.com/category/indirect-elisa