3.6 CELL RECOGNITION AND THE IMMUNE SYSTEM

Identifying self and non-self cells

• Immune system has cells (lymphocytes) to identify pathogens/potentially harmful foreign substances in body & then destroy or neutralise them

How do lymphocytes distinguish between self and non-self cells

• Each cell has specific molecules on its surface to identify it

• Molecules are usually proteins as their 3D- tertiary structure enable unique/identifyable shapes to be made

What happens to a non-self cell?

• If non-self cell is identified, response will be triggered to destroy it

• Different surface molecules enable lymphocytes to identify:

1. Pathogen (bacteria, fungi, or virus such as HIV)

2. Cells from other organisms of the same species (can be harmful for those with organ transplants)

3. Abnormal body cells (such as cancer cells)

4. Toxins (eg. Cholera)

Antigens

• Foreign proteins that generate an immune response by lymphocyte cells when detected in the body

• Located on cell surface

Antigen Variability

• Pathogens DNA can mutate frequently

• If a mutation occurs in the gene which codes for the antigen, the shape of the antigen will change

• Any previous immunity to this antigen (naturally via previous infection or artificially via vaccination) is obsolete/no longer effective as the memory cells stored are of the old shape of the antigen

• This happens in influenza, antigen mutates frequently/quickly and you need a new vaccine each year

Immune response

• If a pathogen gets past chemical and physical barriers of the body, (stomach acid/skin) & enters the blood, then white blood cells are the second line of defense

• WBCs have a specific response (lymphocytes) and a non-specific response (phagocytes)

Phagocytosis (non-specific response)

• Phagocyte is a macrophage (type of WBC) that carries out phagocytosis

• Found in blood and in tissues

• Phagocytosis is a non specific immune response. Any type of non-self cell detected will trigger the same response

Phagocytosis process (8 steps)

1. Pathogens/abnormal cells in blood attract phagocyte and they will move towards these cells

2. Many receptor binding points on the cell surface of phagocytes. Will attach to chemicals/antigens on the pathogen via these receptors

3. Phagocyte changes shape to move around and engulf the pathogen

4. Once engulfed, pathogen is contained within cell in a phagosome vesicle

5. A lysosome within the phagocyte will fuse with the phagosome and release its own contents

6. Lysosome releases lysozyme enzyme into the phagosome. This is a lytic enzyme which hydrolyses the pathogen

7. This destroys the pathogen

8. Soluble products used and absorbed by the phagocyte

T lymphocytes (T cells)

• Lymphocytes are white blood cells involved in the specific immune response

• All lymphocytes are made in the bone marrow, but T cells mature in the thymus

• Cell-mediated response is the response involving t-cells and body cells

Cellular response