Resistance to infection by pathogenic organisms is possible because of:
- Barriers to entry - skin, tears and saliva, mucus and acid in the stomach
- Phagocytosis - by polymorphs and macrophages
- Specific immune responses - antibody-mediated and cell-mediated
B lymphocytes are involved in antibody-mediated immunity
T lymphocytes are responsible for cell-mediated immunity
Antigens are specific molecules, found on the surface of invading pathogens (e.g. bacteria) or on infected cells, which trigger an immune response
Activity of phagocytes
- Pathogen enters
- Changes occur in infected area causing capillaries to become leaky - area becomes swollen
- Phagocytic white blood cells squeeze through the leaky capillary walls and accumulate.
Polymorphs - most common, arrive first
Macrophages - larger and long-lived
- Both types of WBC engulf the bacteria and debris from damaged cells
- Ingested material is enclosed within vacuole
- Lysosomes fuse with vacuole, releasing hydrolytic enzymes which destroy bacteria
- An inactive B lymphocyte with complementary receptors on its surface binds to a foreign (non-self) antigen on a microoganism or virus
- The sensitised B lymphocyte divides by mitosis to form plasma cells and memory cells
- The plasma cells synthesise and secrete antibodies that combine with the antigens on the pathogen and neutralise/destroy them (e.g. agglutination)
- The memory B-cells remain in the body for many years but form plasma cells if the same antigen is detected in the future - providing a more rapid immune response
- A T lymphocyte with complementary receptors binds to a forgein antigen displayed on an infected cell
- The sensitised T lymphocyte divides by mitosis to form helper T-cells, killer T-cells and memory T-cells
- Helper T-cells produce chemicals that stimulate the B lymphocytes, activate killer T-cells and phagocytes
- Killer T-cells release chemical (perforin) that punch holes in the cell-surface membrane, so destroying the infected cells
- Memory T-cells remain in the blood and respond rapidly if the antigen is met again
Active immunity occus when an antigen enters the body and stimulates the body's immune system to produce antibodies and memory cells. Provides long term immunity.
- Natural active immunity - when a person is infected. On first occasion, the person suffers the disease (primary response). On subsequent infection, more rapid immune response due to the action of memory cells (secondary response). Therefore individual is unlikely to suffer the same infection twice.
- Artificial active immunity -involves vaccination. Involves injecting a person with harmless antigens or weakened pathogens, which provokes an immune response. The memory cells produced provide long term immunity. Individual does not suffer the infection at all. Boosters.
Passive immunity occurs when an individual receives antibodies from another source. Only temporary (short term) as antibodies will naturally degrade over time.
- Natural passive immunity - antibodies pass naturally from mother to baby across the placenta and in the mothers' milk
- Artificial passive immunity - when antibodies made in one individual are injected into another person, as a serum.
Transplanting tissue is only possible if the tissue is appropriately matched and medical techniques are used to suppress the immune system.
- Tissue typing - involves the matching of donor and recepient tissue. More likely to occur between close relatives
- X-rays - irradiate bone marrow and lymph tissue to inhibit lymphocyte production, and slow down rejection
- Immunosuppression - use of drugs which inhibit DNA replication, cell division and cloning of lymphocytes, so delay rejection.
Transfusion of blood is only possible if the recipient does not have antibodies complementary to the antigens on the red blood cells of donor.
- Blood group A - contains antigen A and anti-b antibodies
- Blood group B - contains antigen B and anti-a antibodies
- Blood group AB - contians both antigens and neither antibodies - UNIVERSAL RECIPIENT
- Blood group O - contains neither antigens and both antibodies - UNIVERSAL DONOR
- Rhesus positive (Rh+) - contain antigen D, doesn't produce anti-D antibody
- Rhesus negative (Rh-) - doesn't contain antigen D, produces anti-D antibody