C3.2 - Defence against disease

Pathogens

Organism/virus that causes disease by infecting a host.

Skin

Acts as a physical barrier preventing pathogens from entering the body. Made of tightly packed dead cells in the outer layer, which are difficult for microbes to penetrate.

Sebum

An oily substance that inhibits growth of bacteria and fungi by making the skin slightly acidic.

Sweat

Contains antimicrobial substances like enzymes that break down bacterial cell walls.

Mucous membranes

Line body openings that are exposed to the environment such as respiratory tract, digestive tract, urogenital tract. They trap pathogens and particles.

Blood clotting process

When skin is cut or damaged, platelets release clotting factors. They trigger a cascade pathway, resulting in the enzyme thrombin converting fibrinogen into fibrin. Fibrin forms a mesh network that traps erythrocytes and platelets, creating a blood clot. Acts as a temporary seal to prevent further blood loss and blocks entry of pathogens.

Innate immune system

Responds instantly and non specifically to broad categories of pathogens. The response is the same throughout an organism’s lige, it does not improve or adapt with repeated infections. Includes phagocytes.

Phagocytes

Type of white blood cell involved in the body’s first line of defense against infections. Main role is to find, engulf, and destroy invading pathogens like bacteria and viruses.

Amoeboid movement to infection sites

Phagocytes move from the bloodstream into tissues where infection occurs by a special crawling movement called amoeboid movement.

This movement allows them to squeeze through blood vessel walls and travel toward chemical signals released by pathogens or damaged cells.

Recognition of pathogens

Phagocytes recognize pathogens by detecting molecules on the pathogen’s surface that are foreign to the body. These molecules are called antigens or pathogen-associated molecular patterns (PAMPs).

Engulfment by endocytosis

Once a pathogen is recognized, the phagocyte surrounds it with its cell membrane and engulfs it into a vesicle called a phagosome. Uses lysosomes to break down the pathogen.

Lymphocytes

Specialized white blood cells essential for the adaptive immune response, which targets specific pathogens. They circulate in blood and are concentrated in lymph nodes.

B-lymphocytes

Each individual has a huge variety, each capably of producing unique antibody that binds specifically to only one antigen.

Antigens

Molecules that the immune system recognises as foreign and that trigger an immune system. Most are glycoproteins or other proteins and are usually located on the outer surfaces of pathogens.

Antigens on erythrocytes

Antigens are also present on the surface of erythrocytes (red blood cells).

Blood group antigens (like ABO and Rh) can cause immune reactions if blood from one person is transfused into another person with a different blood group.

These antigens can stimulate antibody production, leading to rejection of the transfused blood.

Activation of B lymphocytes

B-cells produce antibodies and develop into memory cells only after activation.

Activation requires two key events:

Direct interaction with the specific antigen, the B-cell’s receptor binds to its matching antigen.

Contact with a helper T-cell that has also been activated by the same antigen.

Role of T helper cells

Helper T-cells recognize the antigen presented by antigen-presenting cells (APCs) and become activated.

Activated helper T-cells then bind to the B-cell presenting the same antigen, releasing signals (cytokines) that stimulate the B-cell.

This cooperation triggers the B-cell to multiply and differentiate into plasma cells that secrete antibodies and memory B-cells for long-term immunity.

Clonal expansion

Once a B-cell is activated (by antigen binding and helper T-cell interaction), it divides by mitosis.

It produces many identical daughter cells (clones) all specific for the same antigen.

Immunity

Ability of the body to eliminate an infectious disease.

Role of memory cells

Immunity is due to long term survival of memory lymphocytes. They help remember the specific antigen of a pathogen encountered during the first infection or vaccination.

HIV

Human immunodeficiency virus, attacks the immune system, specifically helper T lymphocytes, weakening the body’s ability to fight infections.

Fluids that can transmit HIV

Blood

Semen

Vaginal secretions

Breast milk

Rectal fluids

Mechanisms of HIV transmission

Unprotected sexual intercourse, blood to blood contact, mother to child transmission, blood transfusions.

How HIV causes AIDS

HIV infects and kills helper T-lymphocytes, weakening the immune system. The resulting immunodeficiency leads to AIDS, marked by increased vulnerability to infections and disease.

Why antibiotics dont work on viruses

Viruses do not carry out their own metabolic processes; instead, they hijack the host’s (human) cellular machinery to replicate.

Viruses lack cell walls, ribosomes, and metabolic pathways that antibiotics target.

Because viruses replicate inside host cells, antibiotics cannot selectively target viruses without damaging host cells.

Treatment of viral infections typically requires antiviral drugs that specifically interfere with viral replication mechanisms.

Cause of antibiotic resistance

Pathogenic bacteria can evolve resistance to antibiotics through random mutations or by acquiring resistance genes via horizontal gene transfer (e.g., plasmids).

When antibiotics are used, sensitive bacteria are killed, but resistant bacteria survive and multiply, leading to strains resistant to multiple antibiotics (multiresistant bacteria).

Zoonoses

Infectious diseases that can transfer from animals to humans. Eg. Tuberculosis (airborne), Rabies (Bite/saliva), Japanese encephalitis (Mosquito bites), COVID-19 (Respiratory droplets and close contact.

Vaccines

Substances used to stimulate the immune system to develop protection against a specific pathogen.

How vaccines work

The immune system recognizes the antigens from the vaccine as foreign.

This triggers an immune response:

Production of antibodies specific to the pathogen.

Creation of memory cells that remember the pathogen for faster response if exposed again.

Immunity develops without the person becoming ill.

Herd immunity

Happens when a large enough proportion of a population becomes immune to an infectious disease. When people are immune, the disease struggles to spread, protecting those who are not immune.

Percentage change

(New value - Old value/Old value) x 100

Percentage difference

(|Value 1 - Value 2|/Average of values) x 100