C3.2 Defense Against Disease

Disease may be caused by:
genetic factors (mutations / inherited alleles)
environmental factors (toxins, radiation, poor nutrition)
pathogens causing infectious disease

pathogen is a microorganism or infectious agent that causes disease and can be transmitted from one host to another.

The major types of pathogens are:
viruses
bacteria
fungi
protists

Examples:
virus → influenza / HIV
bacterium → cholera
fungus → athlete’s foot
protist → malaria

Observations by Ignaz Semmelweis showed that handwashing by doctors greatly reduced deaths from childbed fever, demonstrating that disease could be spread by contaminated hands. Investigations of cholera outbreaks, especially by John Snow, linked disease transmission to contaminated water. These observations helped establish the importance of hygiene, sanitation, and germ theory in disease control

Primary defense is the first line of non-specific, innate immunity that prevents pathogens from entering the body.

The skin forms a physical barrier made of dead keratinized cells that pathogens cannot easily penetrate.
Sebaceous glands secrete sebum, which lowers skin pH and inhibits bacterial and fungal growth.
Mucous membranes secrete mucus, which traps pathogens and particles.
Mucus may contain lysozyme, an antibacterial enzyme.

Blood clotting:
prevents excessive blood loss
seals the wound
reduces pathogen entry

Platelets adhere to damaged blood vessels and release clotting factors.
These factors initiate a clotting cascade that converts prothrombin to thrombin.
Thrombin converts fibrinogen to fibrin, forming a mesh that traps blood cells and forms a clot.

The blood clotting cascade is triggered when a blood vessel is damaged.
Platelets adhere to the damaged area and release clotting factors.
These factors start a chain reaction that converts inactive prothrombin into active thrombin.
Thrombin acts as an enzyme and converts soluble fibrinogen into insoluble fibrin.
Fibrin forms a mesh of fibers that traps blood cells and platelets, producing a stable blood clot.
This clot reduces blood loss and prevents pathogens from entering the body.

Innate immunity is non-specific, acts rapidly, and includes barriers, phagocytes, and inflammation.

Adaptive immunity is specific to particular antigens, involves B and T lymphocytes, and is slower on first exposure but faster on subsequent exposures due to memory cells.

Phagocytes provide innate immunity by:

1) moving to infection sites
2) recognizing non-self antigens
3) engulfing pathogens by phagocytosis
4) digesting them with lysosomal enzymes

Accumulation of dead phagocytes and pathogens forms pus.

1)recognizes pathogen
2)engulfs it by phagocytosis
3)forms a phagosome
4)lysosome fuses with phagosome
5)enzymes digest pathogen
pathogen fragments may be displayed as antigens on the cell surface

Lymphocytes are white blood cells with:
a large round nucleus
small amount of cytoplasm

They are found mainly in:
lymph nodes
lymphatic vessels
blood

Function:
B lymphocytes produce antibodies
T lymphocytes coordinate immune responses

The specific immune response is the third line of defense in which B cells and T cells respond to specific antigens.

An antibody is a protein produced by B lymphocytes / plasma cells that binds specifically to an antigen.

Lymphocytes recognize specific antigens on the surface of pathogens.
Each B-lymphocyte has receptors with a unique shape that is complementary to one specific antigen.
When the correct antigen binds, the B-cell is activated, usually with the help of helper T-cells.
The activated B-cell undergoes clonal expansion by mitosis and differentiates into plasma cells.
Plasma cells produce large quantities of antibodies with a hypervariable region complementary to the antigen.
These antibodies bind specifically to the pathogen and help neutralize or destroy it.

An antigen is a molecule, usually a protein or glycoprotein, that triggers an immune response.

Antigens are molecules found on the surface of pathogens and are usually proteins, glycoproteins, or large polysaccharides.
A single pathogen may have several different antigens on its surface.
Antibodies have a specific hypervariable region whose shape is complementary to a particular antigen.
Binding occurs due to matching shapes and chemical properties, similar to an enzyme-substrate interaction.
This allows antibodies to bind specifically to the antigen and mark the pathogen for destruction.

ABO blood groups are determined by the antigens present on the surface of red blood cells.

Type A: A antigens, anti-B antibodies
Type B: B antigens, anti-A antibodies
Type AB: both A and B antigens, no anti-A or anti-B antibodies
Type O: no A or B antigens, both anti-A and anti-B antibodies

These antigens determine blood compatibility during transfusions.

If incompatible blood is transfused, antibodies in the recipient’s plasma bind to antigens on the donor red blood cells.
This causes agglutination, where red blood cells clump together.
The clumped cells may block blood vessels and reduce oxygen delivery.
The cells may also undergo hemolysis, where red blood cells burst and release hemoglobin into the plasma. This can lead to serious complications such as kidney damage, shock, and death.

1)A macrophage engulfs a pathogen and displays its antigens on its cell surface.
2)A helper T-cell with a complementary receptor binds to the displayed antigen and becomes activated.
3)The activated helper T-cell then binds to a B-cell that has recognized the same antigen.
4)It releases signaling molecules called cytokines, which activate the B-cell.
5)The activated B-cell then undergoes clonal expansion and differentiates into plasma cells and memory B-cells.

Clonal selection occurs when a B-cell with receptors complementary to a specific antigen is selected and activated.
This B-cell divides repeatedly by mitosis to produce a clone of identical cells.
Some of these cells differentiate into plasma cells, which contain extensive rough endoplasmic reticulum and many ribosomes for rapid antibody production.
Plasma cells secrete large quantities of specific antibodies against the pathogen.

Immunity is the ability of the body to resist or eliminate a specific infectious disease through the action of antibodies and memory cells.

Memory B-cells are long-lived cells formed during the primary immune response.
They remain in the body after the infection has been cleared.
If the same pathogen enters the body again, memory B-cells rapidly recognize the antigen and quickly divide to form plasma cells.
This leads to faster and greater antibody production during the secondary immune response.

The primary immune response occurs when the body encounters a pathogen for the first time. It is slower because specific B-cells must first be selected and activated. Antibody production is lower and takes longer to reach its peak.

The secondary immune response occurs upon re-exposure to the same pathogen. It is faster because memory B-cells are already present. Antibody production is greater, occurs more rapidly, and lasts longer, often preventing symptoms from developing.

HIV is transmitted through the exchange of infected body fluids, including
blood,
semen,
vaginal fluids,
rectal secretions,
breast milk.

Main routes of transmission include:
unprotected sexual intercourse
sharing contaminated needles or syringes
transfusion of infected blood or blood products
mother-to-child transmission during pregnancy, childbirth, or breastfeeding

HIV is a retrovirus with RNA as its genetic material.
It infects and destroys helper T-cells (CD4 cells), which are essential for activating B-cells and coordinating immune responses.
As helper T-cell numbers decrease, the body becomes less able to produce effective antibodies and fight infections.
This leads to increased susceptibility to opportunistic infections and certain cancers such as Kaposi’s sarcoma.
When the immune system becomes severely weakened, the condition is known as acquired immunodeficiency syndrome (AIDS).

Antibiotics are chemicals that kill bacteria or inhibit their growth.

In nature, many antibiotics are produced by microorganisms such as fungi and bacteria to reduce competition for resources. For example, Penicillium fungi produce penicillin.

Medically, antibiotics are used to treat bacterial infections by targeting processes found in prokaryotic cells, such as:

cell wall synthesis
DNA replication
protein synthesis at ribosomes
transcription

Because these targets differ from human cells, antibiotics can kill bacteria without harming body cells.

Antibiotics do not work against viruses because viruses lack their own metabolism and cellular structures such as ribosomes and cell walls.
Viruses reproduce only inside host cells using the host’s enzymes and machinery. T
herefore, antibiotics that target bacterial processes are ineffective against viruses.

Within a bacterial population, some bacteria may possess mutations or genes that provide resistance to an antibiotic.
When the antibiotic is used, susceptible bacteria are killed, while resistant bacteria survive.
These resistant bacteria reproduce and pass the resistance genes to their offspring.
Over time, the frequency of resistance genes increases in the population through natural selection.

Medical causes of antibiotic resistance include:

overprescribing antibiotics
using antibiotics for viral infections
patients not completing the full course
excessive use in livestock farming

These practices increase selection pressure for resistant strains.

Consequences include:

infections becoming harder to treat
longer illness duration
increased hospitalization
greater risk of complications such as septicemia
higher mortality rates

Major routes of pathogen transmission include:
= direct contact → touching, kissing, sexual contact
= droplet transmission → coughing, sneezing, talking
= airborne transmission → aerosols suspended in air
= fomite transmission → contaminated surfaces or objects
= food and water transmission → contaminated food or drinking water
= blood-borne transmission → needles, transfusions
= vector-borne transmission → insects such as mosquitoes
= vertical transmission → mother to child

A zoonosis is an infectious disease that is naturally transmitted from other animals to humans.

An example of a zoonotic disease is rabies, which is transmitted from infected mammals such as dogs through bites or saliva entering broken skin or mucous membranes.

Zoonotic diseases are increasing because humans are coming into closer contact with animals that carry pathogens. This is caused by:

= expansion of human populations into wildlife habitats
= deforestation and habitat destruction
= intensive livestock farming
= wildlife trade and live animal markets
= increased global travel and trade

These factors increase the chances of pathogens crossing from animals to humans. Examples include rabies, avian influenza, and COVID-19.

Immunization is the process of inducing immunity against a disease, usually by administering a vaccine that stimulates the immune system to produce antibodies and memory cells.

Different classes of vaccine active ingredients include:

live attenuated pathogens → weakened forms of the pathogen
inactivated (killed) pathogens
subunit vaccines → specific proteins or antigens from the pathogen
toxoid vaccines → inactivated bacterial toxins
mRNA vaccines → mRNA that codes for a pathogen antigen

Vaccination introduces antigens into the body without causing disease. This triggers a primary immune response, during which specific B-cells and T-cells are activated. Plasma cells produce antibodies, and memory cells are formed.

If the real pathogen later enters the body, memory cells trigger a secondary immune response, which is faster and stronger. Antibodies are produced more rapidly and in greater quantities, preventing symptoms from developing.

Herd immunity occurs when a large proportion of the population is immune to a disease, reducing its spread and indirectly protecting non-immune individuals.

R₀ (basic reproduction number) is the average number of people that one infected person will infect in a completely susceptible population.

Herd immunity limits disease spread because infected individuals are more likely to come into contact with immune people, reducing transmission.

The required percentage of immune individuals can be calculated using:

(1-1/R₀) *100

For example, if R₀ = 15 for measles:

(1-1/15) *100 = 93.3%
So approximately 93% of the population must be immune.

percentage = part/whole *100

Percentage change = (final - inital)/Inital *100

Percentage difference = difference/orginal * 100

if comarparin A to B
A-B/A *100

Pandemic data can be evaluated using:

percentage → proportion of cases or deaths
percentage change → increase or decrease over time
percentage difference → comparison between countries or populations

For example, percentage calculations can be used to compare:

infection rates
death rates
vaccination coverage
changes over time
relationship between wealth and outcomes

These calculations help assess the relative success of countries in controlling a pandemic.