Immune system

This question is testing your basic knowledge of A-level/GCSE content. An excellent candidate will discuss all of the following…

• The immune system recognises antigens, which are molecules on the surface of cells

• Pathogens such as bacteria and viruses have foreign antigens on their surface, which trigger an immune response upon recognition as they differ from the host cell antigens

• The immune system can be divided into the innate and adaptive immune system; the adaptive immune system consists of cell-mediated and humoral immunity

• Cell-mediated immunity uses T cells to kill infected or cancerous cells and target intracellular pathogens such as viruses

• Humoral immunity involves the production of antibodies by B cells, which target specific foreign antigens, as described above

• White blood cells like B/T lymphocytes have specific receptors that bind these foreign antigens,  triggering the response via direct attack (T cells) or antibody production (B cells) 

• Prompt: if candidate is struggling, ask them to think about how an autoimmune response may occur → i.e. they will attack their own self-antigens

Answer:

• When immune cells (i.e. lymphocytes) mature in the bone marrow, they are exposed to self-antigens 

• If these lymphocytes bind strongly to these self-antigens, the cell are destroyed or inactivated (negative selection) by the body’s natural defence mechanisms 

• Prompt: Ask students to suggest how these cells are destroyed → apoptosis 

• For the rest of the cells that do not self-react, they are allowed to mature and enter the circulation → this ensures that only the non self-reactive cells form the body’s immune system

• Self-reactive cells may survive and start attacking the body 

• This would lead to autoimmune disease 

• Prompt: Ask the student to name examples of autoimmune disease → type 1 diabetes, rheumatoid arthritis, multiple sclerosis, psoriasis, Crohn’s disease, Sjogren’s disease, myasthenia gravis etc.

• Excellent candidates may be able to discuss the types of cell affected by these inappropriate autoimmune responses (e.g. pancreatic islet cells in type 1 diabetes, or myelin sheath in multiple sclerosis for example)

• Prompt: ask candidate to think about when an immune system is usually mounted → the body usually only creates an immune response against a trigger of inflammation (e.g. infection, wound)

• The immune system only becomes active when it detects  damage or infection → without a trigger, normal antigens do not trigger inflammation 

• Thus, some form of an inflammatory trigger is are needed alongside the antigen recognition to fully activate lymphocytes 

• In these cases, even if immune cells can bind self- antigens, they may remain inactive without the other signals that warn the immune system of threat