topic 12 - immune systems

3 main phases of immune response

1. Recognition

2. Activation phase

3. Effector phase

Recognition

A pathogen (virus, bacteria, etc) enters the body. Antigen-presenting cells (APCs) such as dendritic cells process the pathogen and present its antigens on their surface using MHC (major histocompatibility complex) molecules. These APCs then travel to lymph noes to interact with naive T cells or B cells.

Organism is capable of discriminating between itself and non-self cells (microbe associated molecular patterns (MAMPs) or pathogen associated molecular patterns (PAMPs)).

Activation phase

Activating a response: immune cells — especially lymphocytes (like T cells and B cells) become activated after recognising a specific antigen.

If a T cell or B cell receptor (TCR/BCR) specifically binds to the presented antigen, activation is triggered.

Effector phase

Defensins in both plants and animals are capable of disrupting the membranes of different microorganisms, primarily bacteria, which results in cell lysis and death.

1. Bacterium engulfed by a macrophage is encased in a vacuole

2. Lysosomes fuse with the vacuole and digest the bacterium

3. Antigens from digested bacterium are presented on the cell surface

Defensins

Are small cysteine-rich antimicrobial peptides that are part of the innate immune system. They help the body fight infections by killing a wide range of pathogens, including bacteria, viruses, and fungi.

Function:

• Insert into microbial membranes, forming pores that cause the pathogen’s contents to leak out and die

• Some defensins can also recruit immune cells, like macrophages and dendritic cells, to the infection site

Innate immunity

Recognition of traits shared by broad ranges of pathogens, using a small set of receptors, rapid response

Barrier defenses: skin, mucous membranes, secretions

Internal defenses: phagocytic cells, antimicrobial proteins, inflammatory response, natural killer cells

Acute defence

Acquired immunity

Recognition of traits specific to particular pathogens, using a vast array of receptors, slower response

Humoural response: antibodies defend against infections in body fluids

Cell-mediated response: Cytotoxic lymphocytes defend against infections in body cells

Long-term defence

Examples of innate immune defences (animals)

Structural barriers:

• skin (epidermis) - tightly packed cells form a physical shield; outer layer contains keratin which resist microbial entry

• cilia in respiratory tract - hair like structures that sweep trapped microbes upward and out of the airways

• tight junctions - between epithelial cells, preventing microbes from slipping between cells

Mucous membranes:

• respiratory mucosa - traps dust, microbes in mucus; cilia help move mucus out of the

• gastrointestinal tract lining - mucus protects intestinal cells from microbes and digestive enzymes

• urogenital tract mucosa - mucus provides a protective barrier against pathogens in the urinary and reproductive tracts

Secretions:

• tears - contain lysosome, break down bacterial cell walls

• saliva - contains enzymes (like lysozyme), and muffins that trap microbes

• sweat - slightly acidic and contains antimicrobial peptides like dermicidin

• gastric juice - very acidic (low pH); kills most ingested pathogens

• vaginal secretions - acidic enviornment inhibits microbial growth; supported by normal flora

Examples of innate immune defences (plants)

Structural barriers:

• cell walls - made of cellulose, lignin and pectin, acts as a rigid barrier to pathogens

• cuticle - waxy, waterproof layers on leaves/stems that blocks pathogen entry and water loss

• bark - protects stems and trunks; a tough outer layer that resists pathogen invasion

• stomata closure - stomata (pores) can close in response to pathogen detection to prevent entry

Mucous-like layers:

• pectin later - found in plant cell walls; forms a gel-like matrix that can restrict pathogen movement

Secretions/Chemical Defense:

Phagocytes in innate immune systems

Detecting, engulfing and destroying invading pathogens

Types of phagocytes:

• neutrophils

• macrophages

• dendritic cells

• monocytes

Phagocytosis

Ingestion: once a pathogen is recognised, the phagocyte engulfs the microbe into a vesicle called a phagosome; phagosome fuses with a lysosome (containing digestive enzymes), forming a phagocytoses

The microbe is destroyed by: enzymatic degradation, reactive oxygen species (ROS), nitric oxide (NO2)

Cytokines production (by phagocytes)

Phagocytes release cytokines and chemokine which promote inflammation, recruit more immune cells to the site of infection, activate the adaptive immune response

Antigen presentation

Some phagocytes (especially dendritic cells and macrophages) also act as antigen-presenting cells (APCs): after digesting. Pathogen, they display fragments (antigens) on their surface using MHC II molecules. This activates T helper cells and links the innate and adaptive immune systems

Neutrophils

Fast-acting, abundant; first responders to infection

Macrophages

Long-lived; engulf pathogens and release cytokines to signal other cells

Dendritic cells

Phagocytose pathogens and present antigens to T cells (bridge to adaptive)

Monocytes

Circulate in blood; differentiate into macrophages or dendritic cells in tissues

Acquired immunity

A specific, long-lasting defence system developed by the body over time through exposure to foreign substances like viruses or bacteria

Cell mediated immunity

Adapted cellular response to prevent infection

Steps in acquired

1. Bacterium or pathogen enters the body

2. Specific white blood cells called macrophages are called in as the first line of defense

3. Via phagocytosis, the macrophages engulf and digest the harmful pathogens that contain unique proteins (antigens)

4. Antigens are fragmented within a phagolysosome and transported to the macrophages surface

5. ^^ referred to as antigen presenting cell (APCs)

6. Other proteins, major histocompatibility class (MHC) molecule embed the antigen fragments for presentation on surface of APC

7. Complex can now be detected by T lymphocytes / T cells (another type of white blood cell) → generate specialised T cells

8. Helper T cells secrete secrete chemicals to stimulate the growth and differentiation of cytotoxic T cells which kill damage cells

9. Memory T cells remember pathogen after the infection has been cleared

Phagocytes

Innate

Engulf and destroy pathogens; include neutrophils, macrophages, and dendritic cells

Detect pathogens using pattern recognition receptors (PRRs)

Engulf pathogens via phagocytosis

Digest them in lysosomes

Secrete cytokines to promote inflammation

Antigen presentation: dendritic cells and macrophages present antigens to activate T cells (linking to adaptive immunity)

Mast cells

Innate

Release histamine and other mediators during allergic and inflammatory responses

Found in tissues, especially near blood vessels and mucosal surfaces

Release histamine, heparin, and other mediators when activated (eg by allergens or pathogens)

Cause vasodilation, increased vascular permeability, and recruitment of immune cells

Play key roles in inflammatory responses, wound healing and allergic reactions

T-helper cell

Acquired

Activate other immune cells (B cells, cytotoxic T cells, macrophages)

Recognise antigens presented on MHC II molecules by antigen presenting cells (APCs)

Coordinate immune responses by secreting cytokines

Activate B cells (to produce antibodies)

Stimulate cytotoxic T cells

Enhance macrophage activity (especially against intracellular pathogens like Mycobacterium)

Cytotoxic T-cell

Acquired

Kill virus-infected and cancerous cells

Recognise infected or abnormal cells displaying antigens on MHC I molecules

Kill target cells: release perforin (form pores in target cells membrane), releasing granzymes (induce apoptosis)

Key defence against virus, tumour cells and some intracellular bacteria

Memory T-cell

Acquired

Cell mediated

Persist after infection to provide faster responses upon re-exposure

Long lived cells formed after an infection or vaccination

Remain in the body and “remember” specific pathogens

Upon re-exposure, they mount a faster and stronger immune response

Suppressor T-cell

Acquired

Suppress or regulate immune responses to prevent autoimmunity

Suppress overactive immune response to prevent autoimmunity

Secrete inhibitory cytokines

Maintain immune tolerance to self-antigens and limit chronic inflammation

Plasma cells

Acquired

Do not divide or live long but produce massive amounts of antibodies

Differentiated B cells that produce antibodies

Differentiated B cells that specialise in antibody production

Each plasma cell produces a large amount of a specific antibody (IgG, IgA, etc)

Antibodies:

1. Neutralise pathogens/toxins

2. Opsonise (mark) microbes for phagocytosis

3. Activate the complement system

Lifespan varies: some are short-lived, others long-lived (in bone marrow)

Memory B cells

Acquired

Humoural

Remember pathogens and mount a faster antibody response upon re-infection

Generated after initial exposure to a pathogen

Don’t produce antibodies immediately but remain dormant in the body

Upon re-infection, they rapidly differentiate into plasma cells to produce large quantities of specific antibodies

Provide faster and stronger antibody responses onset second exposure (basis of vaccines)

B cells

Adaptive - Humoural

Develop and mature in bone marrow

Recognise whole antigens directly using B cell receptors, which are membrane-bound antibodies

T cells

Adaptive - cell-mediated

Develop in bone marrows, mature in thymus

Recognise antigen fragments presented by MHC molecules on the surface of other cells:

MHC I → recognised by cytotoxic T cell

MHC II → recognised by helper T cells