Immunity

Question 1

What are some external defences against pathogens?

Answer 1

• Skin
• HCl in the stomach
• Epithelial (ciliated (cilia) and goblet (excretes mucus)) cells in the airway
• Blood clotting
• Earwax
• Tears
• Saliva

Question 2

What are some internal defence systems?

Answer 2

White blood cells:

• Phagocytes
• Lymphocytes

Question 3

What are antibodies?

Answer 3

Antibodies are glycoprotein molecules that act against specific antigens.

Question 4

What is an immune response?

Answer 4

The response of lymphocytes to the presence of a foreign antigen is known as the immune response. In some cases lymphocytes respond by producing antibodies; in others they respond by killing cells that have become infected by pathogens.

Question 5

What is the difference between antigens and cell surface antigens?

Answer 5

Antigens are a protein that is found on the surface of a pathogen. Cell surface antigens are molecules (proteins) on the surfaces of our cells that are not found in other organisms, or even in other humans.

Question 6

What is the difference between pathogens and antigens?

Answer 6

Antigens are a protein that is found on the surface of a pathogen. Pathogens are harmful microorganisms that can cause diseases.

Question 7

Where does the cells of the immune system originate from?

Answer 7

The cells of the immune system originate from the bone marrow.

Question 8

What are phagocytes?

Answer 8

Phagocytes are produced throughout life in the bone marrow. They are stored there before being distributed around the body in the blood. They are scavengers, removing any dead cells as well as invasive microorganisms.

Question 9

What are neutrophils?

Answer 9

Neutrophils are a kind of phagocyte and form about 60% of the white cells in the blood (Figure 11.3). They travel throughout the body, often leaving the blood by squeezing through the walls of capillaries to ‘patrol’ the tissues. During an infection, neutrophils are released in large numbers from their stores, but they are short-lived cells.

Question 10

What are macrophages?

Answer 10

Macrophages are also phagocytes but are larger thanneutrophils and tend to be found in organs such as the lungs, liver, spleen, kidney and lymph nodes, rather than remaining in the blood. After they are made in the bone marrow, macrophages travel in the blood as monocytes, which develop into macrophages once they leave the blood and settle in the organs, removing any foreign matter
found there. Macrophages are long-lived cells and play a crucial role in initiating immune responses, since they do not destroy pathogens completely, but cut them up to display antigens that can be recognised by lymphocytes.

Question 11

What is phagocytosis in an immune response?

Answer 11

If pathogens invade the body and cause an infection, some of the cells under attack respond by releasing chemicals such as histamine. These, with any chemicals released by the pathogens themselves, attract passing neutrophils to
the site. (This movement towards a chemical stimulus is called chemotaxis.) The neutrophils destroy the pathogens by phagocytosis.

Question 12

How does neutrophils destroy pathogens?

Answer 12

The neutrophils move towards the pathogens, which may be clustered together and covered in antibodies. The antibodies further stimulate the neutrophils to attack the pathogens. This is because neutrophils have receptor proteins on their surfaces that recognise antibody molecules and attach to them. When the neutrophil attaches to the pathogen, the neutrophil’s cell surface membrane engulfs the pathogen and traps it within a phagocytic vacuole in a process called endocytosis. Digestive enzymes are secreted into the phagocytic vacuole, so
destroying the pathogen. Neutrophils have a short life: after killing and digesting
some pathogens, they die. Dead neutrophils often collect at a site of infection to form pus.

Question 13

What do lymphocytes look like?

Answer 13

Lymphocytes are smaller than phagocytes. They have a large nucleus that fills most of the cell.

Question 14

Where are B-lymhocytes found?

Answer 14

B-lymphocytes (B cells) remain in the bone marrow until they are mature and then spread throughout the body, concentrating in lymph nodes and the spleen.

Question 15

Where are T-lymphocytes found?

Answer 15

T-lymphocytes (T cells) leave the bone marrow and collect in the thymus where they mature. The thymus is a gland that lies in the chest just beneath the sternum. It doubles in size between birth and puberty, but after puberty it shrinks.

Question 16

What happens during the maturation process of lymphocytes?

Answer 16

Only mature lymphocytes can carry out immune responses. During the maturation process, many different types of B- and T-lymphocyte develops, perhaps many millions. Each type is specialised to respond to one antigen, giving the immune system as a whole the ability to respond to almost any type of pathogen that enters the body.

Question 17

What happens when lymphocytes mature?

Answer 17

When mature, all these B and T cells circulate between the blood and the lymph. This ensures that they are distributed throughout the body so that they come into contact with any pathogens and with each other.

Question 18

What can mature B lymphocytes do

Answer 18

As each B cell matures, it gains the ability to make just one type of antibody molecule. Many different types of B cell develop in each of us, perhaps as many as 10 million. While B cells are maturing, the genes that code for antibodies are changed in a variety of ways to code for different antibodies. Each cell then divides to give a small number of cells that are able to make the same type of antibody. Each small group of identical cells is called a clone. At this stage, the antibody molecules do not leave the B cell but remain in the cell surface membrane. Here, part of each antibody forms a glycoprotein receptor, which can combine specifically with one type of antigen.
If that antigen enters the body, there will be some mature B cells with cell surface receptors that will recognise it.

Question 19

What happens to B cells during the immune response when an antigen enters the body for the first time?

Answer 19

When the antigen enters the body for the first time the small numbers of B cells with receptors complementary to the antigen are stimulated to divide by mitosis. This stage is known as clonal selection. The small clone of cells divides repeatedly by mitosis in the clonal expansion stage so that huge numbers of identical B cells are produced over a few weeks.

Question 20

What types of B cells are there?

Answer 20

Plasma cells

Memory cells

Question 21

What are the functions of plasma cells?

Answer 21

Plasma cells produce antibody molecules very quickly – up to several thousand a second. Plasma cells secrete antibodies into the blood, lymph or onto the linings of the lungs and the gut. These plasma cells do not live long: after several weeks their numbers decrease. The antibody molecules they have secreted stay in the blood for longer, however, until they too eventually decrease in concentration.

Question 22

What is the purpose of B memory cells?

Answer 22

Memory cells remain circulating in the body for a long time. If the same antigen is reintroduced a few weeks or months after the first infection, memory cells divide rapidly and develop into plasma cells and more memory cells. This is repeated on every subsequent invasion by the same antigen, meaning that the infection can be destroyed and removed before any symptoms of the disease develop.

Question 23

What is the difference between primary and secondary responses?

Answer 23

The first or primary response is slow because, at this stage, there are very few B cells that
are specific to the antigen. The secondary response is faster because there are now many memory cells, which quickly divide and differentiate into plasma cells.

Question 24

What is the structure of antibodies?

Answer 24

Antibodies are all globular glycoproteins with quaternary structure (page 42). They form a group of plasma proteins called immunoglobulins. The basic molecule common to all antibodies consists of four polypeptide chains: two ‘long’ or ‘heavy’ chains and two ‘short’ or ‘light’ chains (Figures 11.9 and 11.10). Disulfide bonds hold the chains together. Each molecule has two identical antigen-binding sites, which are formed by both light and heavy chains. The sequences of amino acids in these regions make the specific three-dimensional shape that binds to just one type of antigen. The antigen-binding sites form the variable region, which is different for each type of antibody molecule produced. The ‘hinge’ region gives the flexibility for the antibody molecule to bind around the antigen.

Question 25

How do antibodies work?

Answer 25

• Some antibodies act as labels to identify antigens as appropriate targets for phagocytes to destroy
• A special group of antibodies are antitoxins which block the toxins released by bacteria
• Antibodies combine with viruses and bacterial toxins preventing them from entering or damaging cells
• Antibodies attach to flagella of bacteria making them less active and easier for phagocytes to engulf
• Antibodies with multiple antigen binding sites cause agglutination (clumping together) of bacteria reducing the chances of spread throughout the body
• Together with other molecules, some antibodies ‘punch’ holes in the cell walls of bacteria, causing them to burst when they absorb water by osmosis
• Antibodies coat bacteria, making it easier for phagocytes to ingest them; phagocytes have receptor proteins for the heavy polypeptide chains of antibodies