Course 25.2Lesson 2 of 3
Acquired immunity is specific to a pathogen and has a memory: a second meeting gives a faster and stronger response. B cells make antibodies, which have two heavy and two light chains. T cells mediate the cell-mediated response. Vaccination uses the memory to protect us before the disease comes.
Try the code and the simulations beside the lesson.
Many children have chickenpox only once. Why? After the first illness the body remembers the virus.
Acquired immunity is the immunity that a person gains during life. It is specific: it is aimed at one pathogen. It has a memory. The first time that the body meets a pathogen, it makes a primary response, which is slow and of low strength. A later meeting with the same pathogen gives a secondary response, or anamnestic response, which is much quicker and stronger.
You should find that the second peak comes sooner after its trigger and is much higher. The model is only a picture. The real curves differ with the antigen and the person.
Two kinds of lymphocytes do the work: the B lymphocytes and the T lymphocytes.
An antibody is a protein made of four peptide chains: two small ones, the light chains, and two longer ones, the heavy chains. We write it as HL. Disulphide bonds join the chains. The molecule has the shape of a letter Y.
At the tips of the two arms are the antigen-binding sites. The parts near the tips are the variable region, which is different in every antibody and fits one antigen. The rest is the constant region. Because each antibody has two arms, it has two binding sites.
There are different types, or classes, of antibody. NCERT names IgA, IgM, IgE and IgG. The facts below are from standard textbooks.
| Class | A fact to remember |
|---|---|
| IgG | the commonest antibody in the blood, and it crosses the placenta |
| IgA | found in colostrum and in secretions such as saliva, tears and mucus |
| IgM | the first antibody that the body makes in an infection |
| IgE | takes part in allergy |
When an organ such as a kidney fails, a donor organ may be used. The body can tell the self from the non-self, and the cell-mediated response attacks a graft as foreign. Doctors therefore match the tissue and the blood group of the donor and the patient. Even then, the patient must take immunosuppressant medicines for life.
A baby gets passive immunity in two ways. The colostrum, the yellowish first milk of the mother, is rich in IgA. The foetus also receives antibodies from its mother through the placenta.
A vaccine is a preparation of antigens. It may hold dead or weakened germs, or only the proteins of a germ. The body meets the antigen, makes antibodies and also forms memory B cells and T cells. When the real pathogen comes later, the memory cells give a quick secondary response, and the person does not fall ill, or only mildly. This is active immunisation.
A person with a deep wound may risk tetanus. A doctor then gives a ready-made antibody, called antitoxin. A snake bite may be treated with an antivenom, which is also a ready-made antibody. These are examples of passive immunisation.
Some vaccines are made by recombinant DNA technology. The vaccine for hepatitis B is made from a protein of the virus, which yeast cells produce.
This part is not in the NEET syllabus, but A-Level and IB ask it.
A vaccine does not only protect the person who gets it. When many people in a town are immune, a case of the disease finds few people to infect, and the disease stops spreading. This is herd immunity.
Suppose that one case infects other people in a town where nobody is immune. For the spread to stop, the share of the town that is immune must be at least
For , the share is , which is per cent.
The program finds the day and the height of each peak in the model of the antibody level.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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