Immunology

Immunology

Definition: the study of the mechanisms by which the body defends itself from attack from foreign material.

Non-self material:  pathogens – any biological agent capable of causing disease to its host

Bacteria (salmonella, Lyme disease)

Viruses (flu virus, HIV)

                Fungi (athlete’s foot, ringworm)

                Parasites/Worms

Non-self material could be cells from other organisms of the same species, toxins (produced by pathogens, e.g. the bacteria that cause cholera), or abnormal cancer cells.

Survival of an organism depends on its ability to distinguish between self and non-self material.

Markers of non-self on the surface of a foreign being are known as antigens.

Antigen: any part of an organism or substance that is recognised as foreign and triggers an immune response and the production of antibodies from lymphocytes.

Lymphocytes aren’t produced in response to an infection, but already exist from the foetal stage. In a foetus, lymphocytes collide only with self material as the foetus is separated from non-self material by the mother and the placenta. Some of the lymphocytes will have receptors which fit the body’s own cells, and these lymphocytes will die or be suppressed. Therefore the remaining lymphocytes will only respond to non-self material.

Pathogen entry routes: insect bites, broken skin, nose, mouth, conjunctiva, anus, placenta, genitalia

Defence Mechanisms

·         Non-specific – response is immediate and is the same for each pathogen (physical barrier e.g. skin, or phagocytosis)

·         Specific – response is slower and specific to each pathogen:

a.       Humoral response – B cells

b.      Cell mediated response – T cells

NON-SPECIFIC RESPONSES:

The skin has sebaceous glands which secrete sebum. This is an antibacterial fatty acid which waterproofs the skin and destroys bacteria.

Mucus lines the airways in order to trap pathogens which are inhaled. These are then removed from the lungs by coughing, sneezing or blowing, or alternatively the mucus is swallowed where it is destroyed, along with the pathogens, by hydrochloric acid in the stomach – cilia are involved in this movement. The hydrochloric acid in the stomach destroys these pathogens as it provides a low pH environment which denatures the enzymes of most pathogens (those in the mucus or in contaminated food or drink).

Tears, saliva and urine have antibacterial qualities and contain enzymes which hydrolyse proteins in pathogens.

Commensal microorganisms are ‘good’ bacteria which help to digest food and acquire nutrients. They also compete against pathogens and prevent them from colonising the body.

 PHAGOCYTOSIS:

1.       A phagocyte is attracted to the pathogen from its chemical products. The cell surface membrane of the phagocyte has receptors with a complementary shape to the antigens of the pathogen – they bind to each other.

2.       The cytoplasm of the phagocyte extends around the bound pathogen, enclosing it in a phagosome.

3.       Lysosomes…