Défense against disease

List three major types of cause of disease.

List three major types of cause of disease.

• Pathogens (infectious agents like bacteria, viruses, fungi, etc.)

• Genetic factors (inherited conditions)

• Environmental factors (e.g., exposure to toxins, poor nutrition)

Define pathogen

A microorganism or other agent (such as viruses) that can cause disease in another organism.

A pathogen is an organism or infectious agent that causes disease. It invades a host and disrupts its normal functioning.

List major pathogen types

• Bacteria: Unicellular, prokaryotic organisms. Ex: Tuberculosis, cholera.

• Viruses: Require living cells to replicate. Ex: COVID-19, influenza.

• Fungi: Eukaryotic organisms, some of which cause disease. Examples: Athlete’s foot, ringworm.

• Protists: Unicellular, eukaryotic organisms. Ex: Malaria, toxoplasmosis.

• Prions: Infectious proteins that cause neurodegenerative diseases. Ex: Bovine spongiform encephalopathy (mad cow disease).

Define “primary defense.”

refers to the body's first line of defense, which prevents pathogens from entering the body, mainly through physical and chemical barriers such as skin and mucous membranes.

The first line of defence by the body is non-specific and prevents the entry of any potential pathogen into the body.

Outline the role of skin in the defence against pathogens.

• Skin acts as a physical barrier with layers of dead, keratinized cells in the epidermis.

• Constant shedding of outer skin layers removes attached microbes.

• Skin secretes sweat and sebum, lowering pH to around 4-5, inhibiting bacterial growth.

Outline the role of sebaceous glands in the defence against pathogens.

• Sebaceous glands secrete sebum containing fatty acids and lipids that lower skin pH.

• Antimicrobial properties in sebum prevent bacterial and fungal infections.

• Sebum contains antimicrobial peptides that directly neutralize harmful microbes.

Outline the role of mucous membranes in the defence against pathogens.

• Mucous membranes line internal body cavities such as the respiratory, digestive, and urogenital tracts, secreting mucus that traps pathogens, preventing them from reaching deeper tissues.

• Mucus contains the enzyme lysozyme, which breaks down bacterial cell walls, effectively destroying bacteria that come in contact with it.

• In the respiratory tract, cilia (tiny hair-like structures) move the mucus, along with trapped pathogens, toward the throat, where it can be expelled by coughing or sneezing, thus physically removing microbes from the body.

State two benefits of blood clotting when skin is cut.

• Prevents excessive blood loss.

• Blocks the entry of pathogens at the injury site.

Outline two roles of platelets in the blood clotting cascade.

• Platelets accumulate at the site of injury and form a plug to seal the wound.

• They release clotting factors, triggering the clotting cascade that leads to the formation of a stable clot.

Describe the blood clotting cascade, including the role of platelets, clotting factors, prothrombin, thrombin, fibrinogen and fibrin.

• Platelets rush to the injury site, clumping together to form a plug, acting as a physical barrier that immediately reduces bleeding.

• Platelets release clotting factors like thromboplastin and calcium ions, initiating the clotting cascade.

• Clotting factors activate prothrombin, converting it into thrombin. Thrombin then converts fibrinogen into fibrin.

• Fibrin forms a mesh that traps red blood cells and platelets, creating a stable clot that seals the wound and prevents pathogen entry.

Distinguish between innate and adaptive immunity, including the types of cells and timing of response to infection.

		FeatureInnate Immune SystemAdaptive Immune System
Timing of Response	Immediate (minutes to hours)	Delayed (days to respond)
Specificity	Non-specific, same response to all pathogens	Highly specific, targets particular pathogens (e.g., viruses, bacteria)
Cells Involved	Phagocytes (e.g., macrophages, neutrophils, monocytes)	Lymphocytes (B cells, T cells)
Memory	No memory, same response each time	Has memory, quicker and stronger response on re-exposure
Defense Mechanism	Physical barriers (skin, mucous membranes), inflammation, phagocytosis	Antibody production, cytotoxic T cells
Duration of Response	Short-term, rapid but limited in duration	Long-term, can last years due to immunological memory
Communication with Other System	Communicates and activates adaptive immune system if unable to eliminate pathogen	Relies on innate system to activate and present pathogens for targeted attack

Function of phagocytic white blood cells in defence

• Phagocytes like neutrophils and macrophages are crucial in the non-specific immune response, targeting a wide range of pathogens.

• They recognize pathogens through receptors on their surface, which bind to the pathogen, initiating phagocytosis.

• Phagocytes engulf and digest pathogens, eliminating them from the body before they can spread or cause harm.

Process by which a macrophage destroys a pathogen

• Recognition and binding: The macrophage identifies the pathogen by binding to it through receptors on its plasma membrane.

• Engulfment: The macrophage extends pseudopodia around the pathogen, engulfing it into a vesicle known as a phagosome.

• Phagosome maturation: The phagosome fuses with lysosomes, forming a phagolysosome that contains digestive enzymes.

• Digestion: The enzymes break down the pathogen’s components, killing it.

• Expulsion: The digested materials are either used by the macrophage or expelled from the cell as waste.

What are the professional phagocytes of the body?

• macrophage

• neutrophils

• Monocytes

Structure and Function of Lymphocyte Cells

• B-Lymphocytes (B-Cells): Develop in the bone marrow. Produce antibodies that bind to specific pathogens. Key role in humoral immunity.

• T-Lymphocytes (T-Cells): Mature in the thymus. Two types: Helper T-cells, which activate other immune cells, and Cytotoxic T-cells, which kill infected cells. Involved in cell-mediated immunity.

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• B-Lymphocytes (B-Cells):

  • B-lymphocytes are a type of white blood cell that develop in the bone marrow and play a critical role in the adaptive immune response.

  • Once activated by a specific antigen, B-cells differentiate into plasma cells, which are specialized to produce large quantities of antibodies. These antibodies are highly specific and bind to antigens on the pathogen, marking it for destruction by other immune cells or neutralizing its harmful effects.

  • B-cells are responsible for humoral immunity, which refers to the production of antibodies that circulate in the bloodstream and lymph. Humoral immunity is essential for targeting pathogens in the blood and extracellular fluids.

  • B-cells also generate memory B-cells after the initial infection, allowing for a faster and more efficient immune response upon future exposures to the same pathogen.

• T-Lymphocytes (T-Cells):

  • T-lymphocytes mature in the thymus and are also involved in the adaptive immune system. T-cells are primarily responsible for cell-mediated immunity, which involves the direct destruction of infected or abnormal cells.

  • There are two main types of T-cells:

    1. Helper T-cells (CD4+ cells): These cells play a central role in orchestrating the immune response. They activate B-cells, cytotoxic T-cells, and macrophages by releasing signaling molecules called cytokines. Helper T-cells are essential for the initiation and coordination of the immune response.

    2. Cytotoxic T-cells (CD8+ cells): These cells directly attack and kill cells that are infected by viruses or have become cancerous. They recognize infected cells through antigens presented on the cell surface and release enzymes that induce cell death (apoptosis).

  • Like B-cells, T-cells also form memory T-cells, which remain in the body after an infection and respond rapidly if the same antigen is encountered again, providing long-term immunity.

• Interaction Between B- and T-Cells:

  • Helper T-cells activate B-cells by recognizing the same antigen and secreting cytokines that stimulate B-cells to proliferate and differentiate into plasma cells.

  • This cooperation between B- and T-cells ensures a coordinated and highly specific immune response tailored to eliminating pathogens effectively.

Location of lymphocytes in the body

blood, lymphatic vessels, and secondary lymphoid organs such as lymph nodes, spleen, and tonsils.

Definition of “Specific” in Immune Response:

"Specific" means that lymphocytes recognize and target particular antigens on pathogens, leading to an immune response tailored to that specific pathogen.

Definition of “Antibody”:

Antibodies are Y-shaped proteins produced by B-cells. They bind to specific antigens on pathogens, neutralizing them or marking them for destruction.

Role of Lymphocytes in Producing Antibodies:

B-Cells recognize a specific antigen, become activated, and differentiate into plasma cells, which secrete large amounts of antibodies. These antibodies bind to the antigen and help eliminate the pathogen.

Definition of Antigen:

An antigen is any substance that triggers an immune response by stimulating the production of antibodies. Antigens are usually proteins found on the surface of pathogens.

Antigens are typically proteins, glycoproteins, or lipoproteins found on the surface of pathogens. They are recognized by immune cells due to their unique molecular structure.

Cause and Consequence of Antibody Binding to an Antigen:

• Cause: Antibodies bind to antigens due to complementary shapes, like a lock and key, targeting specific pathogens.

• Consequence 1 – Neutralization: Antibodies block pathogen entry into host cells by binding to key sites, preventing infection.

• Consequence 2 – Opsonization: Antibodies mark pathogens for destruction by phagocytes, making them easier to recognize and engulf.

• Consequence 3 – Complement Activation: Antigen-antibody complexes trigger complement system, which punches holes in the pathogen's membrane, leading to membrane attack complexes that destroy pathogens.

Difference Between ABO Blood Antigens:

Blood groups are determined by the presence of specific antigens on the surface of red blood cells:

• Group A: Has A antigens.

• Group B: Has B antigens.

• Group AB: Has both A and B antigens.

• Group O: Has no A or B antigens.

Consequence of Mismatched Blood Transfusions:

When incompatible blood is transfused, the recipient’s antibodies bind to the donor’s RBC antigens, leading to agglutination (clumping of RBCs) - clumping blocks blood vessels and disrupts normal blood flow.

and hemolysis (rupturing of RBCs) releasing hemoglobin into the bloodstream. Free hemoglobin can damage organs, particularly the kidneys, leading to kidney failure, shock, and in severe cases, death.