Specific Defence Mechanism: MDCAT Biology notes
Specific Defence Mechanism for MDCAT: B and T lymphocytes, humoral vs cell-mediated immunity, antibody structure, active vs passive immunity and vaccines.
Non-specific vs specific defence
Non-specific (innate) defence acts against any invader. Its first line is the physical barrier of the skin and mucous membranes. The second line includes phagocytes, natural killer cells, interferons and inflammation. Interferons are proteins released by virus-infected cells; they make neighbouring cells resist the virus and activate natural killer cells. In inflammation, damaged tissue and mast cells release histamine (dilates vessels, causing redness and swelling) and bradykinin (makes capillaries leaky so fluid leaks into the tissue).
Specific defence (the immune system) recognises a particular antigen, a foreign molecule that triggers antibody production, and remembers it. It is carried out by lymphocytes.
B and T lymphocytes
| Feature | B lymphocytes | T lymphocytes |
|---|---|---|
| Formed in | Bone marrow | Bone marrow |
| Mature in | Bone marrow (named after the bursa of Fabricius in birds) | Thymus (in the mediastinum of the upper thorax) |
| Response | Humoral: antibodies released into plasma, lymph and tissue fluid | Cell-mediated: cells attack directly |
| Main targets | Bacteria, toxins, viruses in body fluids | Infected cells, cancer cells, transplanted organs |
An activated B cell divides into plasma cells, which secrete antibodies, and memory cells.
Types of T cell
- Helper T cells: recognise antigen and release interleukins that stimulate division of both B cells and other T cells. They are needed to activate B cells. HIV infects and destroys helper T cells.
- Cytotoxic (killer) T cells: kill infected or foreign cells, e.g. by releasing perforin.
- Suppressor T cells: damp down the immune response when it is over.
- Memory T cells: give a faster response on re-exposure.
Graft rejection is a cell-mediated response, so after organ transplants T-cell activity is suppressed with drugs. Swollen lymph nodes during a throat infection show an active immune response as lymphocytes multiply.
Antibody structure and action
Antibodies (immunoglobulins) are Y-shaped proteins made of four polypeptide chains: two identical heavy and two identical light chains joined by disulphide bonds. The tips of both heavy and light chains have variable regions that form the antigen-binding sites; the rest is the constant region.
Antibodies act by neutralising toxins, agglutinating or precipitating antigens, and marking them to enhance phagocytosis. They do not secrete cytokines; cytokines come from cells.
Active and passive immunity
| Type | Natural | Artificial |
|---|---|---|
| Active (body makes its own antibodies; memory; long lasting, slow to develop) | Recovering from an infection such as chickenpox | Vaccination (polio drops, MMR shots) |
| Passive (ready-made antibodies received; immediate but short-lived) | Antibodies from mother via placenta or breast milk | Antivenom serum, anti-tetanus or anti-rabies serum, convalescent plasma |
A vaccine contains antigens (killed or weakened microbes or their products), not antibodies. For active infections such as tetanus, rabies or snake bite, passive immunisation is used because it acts at once.
Autoimmunity
In autoimmune disease the immune system fails to distinguish self from non-self and attacks the body's own tissue ("self against self").
Common MDCAT traps
- Antibodies are made by B cells (plasma cells), not T cells.
- Vaccination is artificial active immunity; antivenom is artificial passive.
- Only active immunity is long-lasting.
- Helper T cells, not cytotoxic cells, activate B cells and are HIV's target.
- Antibiotics kill bacteria but do not build immunity; vaccines do.
Quick revision
- Humoral = B cells and antibodies; cell-mediated = T cells.
- B cells mature in bone marrow; T cells in thymus.
- Antibody = 2 heavy + 2 light chains, disulphide bonds.
- Interferons fight viruses.
- Transplant rejection is cell-mediated.