Understanding the Body’s Defense Mechanisms
Mechanisms of Defense (Barriers)
Non-Specific Barriers
These are congenital. We are born with them, and they do not discriminate between pathogens and non-pathogens. They have no memory, which means they do not recognize antigens and are not reinforced after infection.
- The skin is both a mechanical and chemical barrier. The chemical barrier function depends on two glands: sebaceous and sweat glands. Both secrete a fluid that gives the skin a slightly acidic pH, which is very effective against external germs, e.g., staphylococci in acne.
- Mucous membranes are highly vascular soft tissues that coat any surface or internal cavity. They secrete mucus that prevents the binding of bacteria. The mucus contains lysozyme (enzyme), which has bactericidal action.
- Body fluids such as saliva and tears have a scanning function and contain bactericidal enzymes. Lactoperoxidase is an enzyme that acts as a scanner.
- Bacterial flora are symbiotic bacteria that live with us, and we both benefit.
Their function is to compete with pathogens for the same resources. Example: Candida.
Cellular Barrier
Macrophages are cells derived from bone marrow, like any blood cell. They can leave blood vessels and move between tissues. They are very large and have basically three functions:
- Eliminate waste from any tissue, e.g., reabsorb pus.
- Detect antigens and mark them, making them accessible to antibodies.
- Awaken the immune and inflammatory responses.
Inflammatory Response
If a tissue is broken, with continuous external bleeding and an open wound, the response is immediate, increasing blood circulation to close the bleeding and bringing more blood plasma with white blood cells and antibodies.
This prevents the entry of any pathogens. The pain is caused by prostaglandins that stimulate nerve endings. Fever is caused by pyrogens released by macrophages. These pyrogens travel in the blood to the brain and stimulate the hypothalamus, where the central control of body temperature is located. Fever is a defense mechanism. Histamines, the same substances that cause allergies, are responsible for vasodilation.
Tissue Damage => Histamine Release => Vasodilation
Specific Barriers
Immunity: We are immune to something when we have specific antibodies that inhibit the antigen.
There are two types:
- Active: The host produces antibodies. It can be acquired in two ways:
- Artificial: A vaccine is supplied to the host, and it makes antibodies. [A vaccine contains antigens that are dead, weakened, or non-toxic, but they cause an immune response.]
- Natural: The individual has to suffer from the disease.
- Passive: Antibodies come from outside the host. There are two routes:
- Artificial: The patient is provided with a serum containing specific antibodies. This is an antidote.
- Natural or Congenital: The mother transfers antibodies to the fetus through the placenta, which are for life. This is reinforced by breast milk.
Antigen: The source can be biotic or abiotic, but they are always molecules that provoke an immune response in the patient.
Antibody: Proteins that fall into the family of immunoglobulins. The tertiary or humoral barrier depends on them, and it is the most effective of all our defensive barriers. They are both individual and antigen-specific.
Antibody Reactions
Three types:
- Agglutination: The antibody binds antigens together so they are accessible to macrophages. E.g., Hemagglutination happens when incompatible blood is introduced.
- Neutralization: Affects mostly viruses and bacteria, preventing them from spreading.
- Opsonization: In this case, it acts on tetanus toxin.
Autoimmunity
It is a disorder of uncertain origin, presumably with a genetic basis. The individual’s defenses attack their own body, reacting against it. It can be mitigated with immunosuppression, i.e., decreasing the patient’s defenses, as in a transplant.
Ex: Lupus erythematosus and pernicious anemia.
Immunodeficiency
These are disorders that have an organic or incidental basis. Examples include AIDS and transplant recipients. It manifests as a reduction in the individual’s defenses.
Allergy
Hypersensitivity to antigens that should not be foreign. It manifests with a very violent reaction of the individual. Typically, it is localized, but when extended, it can cause anaphylactic shock.
Rejection
It is a response to a transplanted tissue that is foreign.