Key Area 6
(a) Lymphocytes
Something to Read...
Open the document below and read the mandatory course notes provided by SQA for this Key Area. You may wish to use your home learning time to copy these notes into a jotter or choose a digital presentation format that suits your learning style.
Something to Watch...
Something to look at...
What's an antigen?
Antigens are molecules, often proteins located on the surface of cells that trigger specific immune response. These antigens also become the target for many vaccine development programmes.
The SARS-CoV-2 virus particle, responsible for causing COVID-19, contains four major structural proteins: spike (S), membrane (M), envelope (E) and nucleocapsid (N). The S protein is the main protein used as a target in COVID-19 vaccines.
Something to look at...
Clonal Selection
In the red bone marrow, blood stem cells divide to produce daughter cells. During differentiation, immature lymphocytes each develop a different antigen receptor on their membranes. The first image, shown right, demonstrates that mammals contain many different lymphocytes, each possessing a receptor on its surface which can potentially recognise a pathogen antigen.
Immature lymphocytes which carry a receptor that will bind with an antigen from the body's own tissues are destroyed in the bone marrow. Lymphocytes carrying other antigen receptors are released from the bone marrow and move through the circulatory system to the lymph glands and divide.
Most lymphocytes never encounter an antigen to match their receptor. However, binding of an antigen to a lymphocyte's receptor "selects" that lymphocyte to then divide and produce a clonal population of this lymphocyte. Some selected lymphocytes will produce antibodies (labelled as "memory cells" in the diagram below), others can induce apoptosis in parasite-infected cells (labelled as "plasma cells" in the diagram below).
B lymphocytes
An antibody is a Y-shaped protein molecule produced by B lymphocytes, in response to a specific antigen inside the body. This antigen is recognised as foreign/non-self and can bind to the receptor sites positioned at the ends of the shorter arms of the antibody, which are genetically variable.
Once bound to the antibody, an antigen-antibody complex. This is called agglutination. The large mass is recognised by phagocytes and destroyed by phagocytosis.
In the diagram (left), the word "epitope" is used. You do not need to know this word; however, an epitope is the part of the antigen that is recognised specifically by the antibody.
B lymphocytes can respond to antigens on substances that are harmless on the body, e.g. pollen. This hypersensitive response is called an allergic reaction. A classic example is hayfever.
In these cases, the diagram (right) does not apply!
T Lymphocytes
T lymphocytes destroy infected body cells by recognising antigens of the pathogen on the cell membrane. The lymphocytes attach onto the infected cells and release proteins. These proteins diffuse into the infected cells causing production of self-destructive enzymes. This triggers a process called apoptosis, or programmed cell death.
The remains of the cell are then removed by phagocytosis. T lymphocytes can normally distinguish between self-antigens on the body’s own cells and non-self-antigens on infected cells.
This process is shown right.
Autoimmune disease
Failure of the regulation of the immune system leads to T lymphocytes responding to self-antigens. This causes autoimmune diseases. In autoimmunity, the T lymphocytes attack the body’s own cells. This causes autoimmune diseases such as type 1 diabetes and rheumatoid arthritis.
You are now ready for your in-class lesson on elevated glucose levels.
You are now ready to move onto Key Area 8b: insulin, glucagon and adrenaline.
