C3.2 Defence Against Disease

Pathogen

Organisms that cause disease

Pathogens include:

Certain species of bacteria, certain species of fungi, certain species of protists, viruses, prions

Skin

Composed of 3 layers: epidermis, dermis, and hypodermis

Epidermis

The surface layers consist of dead cells with keratin deposits, forming a tough barrier that prevents the entry of microorganisms

Sebaceous glands

They produce oils that keep the skin at a slightly lower pH which prevents the growth of bacteria (chemical barrier)

Mucous membranes

They produce mucus

Mucus

Traps pathogens and contains lysozyme. It is expelled from the body by coughing or sneezing

Lysozyme

An enzyme which attacks bacterial cell walls and kills the bacteria

Process of blood clotting

Activation and accumulation of platelets at the site of injury

The platelets form a plug sealing the injury

The platelets, along with the injured tissues, release clotting factors (most of these clotting factors are normally present in their inactive form)

Prothrombin converts into its active state, thrombin

Thrombin converts fibrinogen to insoluble fibrin

Fibrin forms a mesh over the wound that entraps more platelets and red blood cells, resulting in a blood clot

Innate immunity

Immunity that is present from birth, responsible for non-specific defence responses

Adaptive immune response

Initiated when the innate immune system is unable to control the pathogens, has features such as specificity and memory

Phagocytosis

The process in which any foreign material, including pathogens, is ingested and digested

Phagocytes

A type of immune cell that can surround and kill microorganisms, ingest foreign material, and remove dead cells

Lymphocytes

Circulate in the blood and are contained in the lymph nodes

T-lymphocytes (T-cells)

Mature in the thymus

B-lymphocytes (B-cells)

Mature in the bone marrow. Each cell is programmed to make one specific antibody

Antigen

Any substance that triggers the immune system, resulting in the production of antibodies against it. Found on the surface of a pathogen

Helper T-cells

Activate other immune cells to fight it

Cytotoxic T-cells

Kill infected cells

Activation of B-cells

Occurs only when both contact with helper T-cells and direct interaction with the specific antigen take place

Activation of helper T-cells

After phagocytes engulf a pathogen, they present its antigens on their surface, which T-cells detect, activating them

Multiplication of B-cells

The activated B-cell undergoes repeated mitosis to form plasma cells and memory cells.

Plasma cells

Each cell produces and releases antibody molecules that are specific to the same antigen

Immunity

The ability to eliminate an infectious disease from the body

Transmission of HIV

Sexual intercourse with an infected person, transfusion of infected blood, sharing needles or syringes, from mother to child during childbirth or breastfeeding

Immunodeficiency

The decreased ability of the body to fight infections and other diseases

Effect of HIV on the immune system

Attacks the helper T-cells, leading to a depletion of helper T-cells, affecting the activation of cytotoxic T-cells and the B-cells

Antibiotics

Chemicals that block processes occurring in bacteria but not in eukaryotic cells

Antibiotics and viruses

Viruses are considered to be non-living, so they do not have metabolic processes or structures that can be disrupted by antibiotics

Antibiotic resistance

When bacteria develops defences against the antibiotics designed to kill them

Zoonoses

Any disease or infection that is naturally transmissible from vertebrate animals to humans

Examples of zoonoses

Tuberculosis, rabies, Japanese encephalitis, COVID-19, brucellosis, anthrax, Ebola

Immunisation

Is the process by which our bodies develop immunity and become resistant to that disease

Herd immunity

Achieved when a large portion of the population has already contracted the disease before or is vaccinated against it, greatly impeding its spread