Antigens and the immune system

1. Pathogens that get past the skin, mucus and other barriers enter the tissues and the blood, where the immune system is the next line of defence.
2. The immune system is made up of the white blood cells and the proteins they release, and its job is to find and destroy pathogens inside the body.
3. Phagocytes give the first, non-specific part of this response by engulfing and digesting any pathogen they meet.
4. Lymphocytes give the specific immune response, which targets one particular type of pathogen and destroys it far more effectively.
5. To respond to a pathogen, the immune system must first recognise it as foreign.
6. Every cell has molecules called antigens on its surface, and these are usually proteins with a particular three-dimensional shape.
7. The antigens on your own cells are recognised as self, so lymphocytes do not attack them.
8. The antigens on a pathogen have different shapes, so they are recognised as non-self, and this is what triggers the response.
9. Each type of pathogen carries its own unique antigens, so the measles virus, the chickenpox virus and the bacterium that causes tuberculosis each trigger a different response.

How antibodies destroy pathogens

1. The body contains millions of different lymphocytes, and each one carries receptors of one shape and can make antibodies of only that shape.
2. When a pathogen enters the body, only the lymphocytes whose antibodies are complementary to its antigens can bind to it.
3. The specific immune response then follows a sequence of steps.
   1. The lymphocyte with the matching antibody binds to the antigen and is activated.
   2. The activated lymphocyte divides rapidly by mitosis, producing a clone of many genetically identical lymphocytes.
   3. The cloned lymphocytes release large numbers of antibodies into the blood, all with the same shape.
   4. The antibodies bind to the antigens on the surface of the pathogens.
4. An antibody is a Y-shaped protein with two identical binding sites, one at the tip of each arm.
5. Each binding site has a shape that is complementary to one antigen, so the antibody fits that antigen and no other.
6. Once they have bound, antibodies destroy pathogens in several ways.
   1. Because each antibody has two binding sites, it can attach to two pathogens at once, and this causes agglutination, the clumping of many pathogens together.
   2. Clumped pathogens cannot spread through the body as easily, and a phagocyte can engulf a whole clump at once.
   3. Bound antibodies act as markers that tag the pathogen, so phagocytes recognise and engulf it more quickly.
   4. Antibodies that cover the antigens of a virus stop it attaching to and entering host cells, so it cannot reproduce.
7. The phagocytes then engulf and digest the tagged and clumped pathogens, so the two types of white blood cell work together.

Antitoxins

1. Some bacteria cause the symptoms of disease by releasing toxins, which are poisonous substances that damage body cells.
2. The toxins released by the bacteria that cause tetanus and diphtheria are examples, and these toxins can be fatal even when the number of bacteria is small.
3. Lymphocytes respond by producing antitoxins, which are proteins with a shape complementary to one particular toxin.
4. An antitoxin binds to the toxin and neutralises it, so the toxin can no longer damage cells.
5. Antitoxins deal with the toxin while antibodies and phagocytes deal with the bacteria that are releasing it.

Memory cells and immunity

1. When a lymphocyte is activated and divides, some of the cells in the clone do not release antibodies straight away but become memory cells.
2. Memory cells stay in the blood for many years, sometimes for life, after the infection has been cleared.
3. If the same pathogen enters the body again, the memory cells recognise its antigens immediately.
4. They divide rapidly and produce antibodies much faster and in much larger amounts than the first time.
5. The pathogens are destroyed before they can reproduce enough to cause symptoms, so the person does not become ill.
6. A person who has memory cells for a pathogen in this way has immunity to the disease that it causes.
7. Immunity is specific to one pathogen, so having had chickenpox gives immunity to chickenpox but no protection against measles.

Primary and secondary responses

1. The response to the first exposure to a pathogen is called the primary immune response.
2. It is slow, because only a few lymphocytes have the right antibody shape and they must divide many times before much antibody is made.
3. There is a delay of several days before antibody appears in the blood, and during this time the pathogens reproduce and the person develops symptoms.
4. The concentration of antibody peaks after about one to two weeks and then falls as the pathogen is cleared.
5. The response to a later exposure to the same pathogen is called the secondary immune response.
6. It is faster, because memory cells are already present and start producing antibody within a day or two.
7. It is larger, because there are many more memory cells than there were matching lymphocytes at the start, so the peak concentration of antibody is often ten times higher or more.
8. It lasts longer, so antibody stays in the blood at a high concentration for longer.
9. On a graph of antibody concentration against time, the secondary response therefore appears as a curve that rises sooner, climbs more steeply and reaches a much higher peak than the primary response.