#2 Intro to immune function (acquired)

What are the 3 principle mechanisms of the immune system?

• Physical barriers are always present

• Innate immune responses act very fast

• Adaptive (acquired) immune responses take time to develop

Complement (C’)

A group of proteins in blood serum and on cell membranes that act in enzyme cascades to help kill pathogens

• involved in both innate and acquired immunity

How does the complement system kill pathogens?

• Enhances phagocytosis

• Causes lysis of bacterial cell walls

• Encourages inflammation

What is the key difference between antigen and antibody?

• Antigens stimulate the immune response

• Antibodies are produced in response to antigens

Innate immune system

A fast, non-specific defense system that is present at birth

Acquired immune system

A slower, highly specific immune system that develops after exposure to antigens

What role do physical barriers play in immunity?

Passive, non-specific, first line of defense that is always ready

How fast does the acquired immune system respond?

• First exposure: ~10–14 days

• Subsequent exposures: much faster

Why does the first infection act like a vaccine?

Because it creates immune memory for faster future responses

What does “specificity” mean in acquired immunity?

Immune cells recognize specific antigen structures

Heterogeneity

The presence of >10⁶ different B- and T-cell clones with unique receptors

What is clonal selection theory?

Only lymphocytes whose receptors bind to an antigen are activated and proliferate.

• specifically recognize and fight pathogens by selecting and expanding lymphocytes (B/T cells) already possessing receptors for a specific pathogen

Why it matters: Ensures specificity and efficiency

Immune memory

long-lived memory cells that elicit a faster and stronger immune response upon second exposure

What are the two components of acquired immunity?

1. Cell-mediated immunity (T-cells)

2. Humoral immunity (B-cells and antibodies)

Cellular immunity

Immune responses carried out by cells that directly attack infected or abnormal cells

Humoral immunity

Immune responses involving antibodies circulating in serum

Antigen Presentation

The process where antigens are broken down and displayed on MHC molecules on cell surfaces to activate T-cells

Which cells are key antigen-presenting cells (APCs)?

• Monocytes/macrophages

• Dendritic cells

• B-cells

MHC

A group of membrane proteins that bind antigens and present them to T-cells

What do monocytes do?

Phagocytize pathogens and present antigens

What turns monocytes into macrophages?

Migration from bloodstream into tissues

What stimulates monocytes/macrophages to phagocytize?

Opsonins (antibodies, complement) and cytokines

What do helper T-cells do?

• Recognize presented antigens

• Release cytokines

• Activate effector immune cells

Where are lymphocytes educated?

• T-cells: thymus

• B-cells: bone marrow (mammals) or bursa (birds)

Hematopoietic maturation

The process of immune cell education and specialization

What do T-lymphocytes (T-cells) do?

• Kill infected cells directly

• Release cytokines to influence immune responses

Clonal expansion

Rapid multiplication of antigen-specific T-cells after activation

What do B-lymphocytes do?

Produce antibodies in response to antigens

Plasma cell

An activated B-cell that secretes large quantities of antibodies

Immunoglobulins (Ig)

Serum proteins produced in response to antigens

• antibodies

What is the general structure of antibodies?

• Y-shaped

• Two variable (binding) regions

• Hinge region for flexibility

What are the Five Classes of Antibodies?

• IgM

• IgG

• IgA

• IgE

• IgD

What is IgM?

• First antibody produced during infection

• Coated with antibodies-> Opsonins

• Activates complement

What is the shape of IgM?

Pentamer

What is IgG?

• Most abundant antibody

• Crosses placenta (Get from mom until build up own supply)

• Effective against bacteria in bloodstream

What is the shape of IgG?

Monomer

What is IgA?

• a barrier antibody found in mucous membranes

• Found in the linings of the intestines, respiratory tracts, & urogenital tracts

  • Ex:

    • saliva

    • tears

    • milk

    • colostrum

      • Transfers IgA to offspring→ helps them survive in the beginning

What is the shape of IgA?

Dimer

What is IgE?

• Helps to initiate inflammatory responses

• Binds to surface of mast cells and basophils and cause release of histamine

What is the shape of IgE?

monomer

What is IgE responsible for?

• Allergic reactions and antiparasitic defense

• Triggers histamine release

What is IgD?

• Poorly understood

• involved in B-cell activation and mucosal immunity

What is the shape of IgD?

monomer

What are the main cell types that participate in the immune response?

• B-cell

• Dendritic cell 

• Macrophage

What is the function of dendritic cells?

• Activate T- and B-cells in lymph nodes

• Capture

  • (phagocytosis)

• Process

  • (break-down and attach antigen to MHCII molecules on the internal phagosome membranes)

• Present antigen on their cell membranes to other leukocytes

  • (this is an important antigen presenting cell)

Where are dendritic cells found?

Found throughout all tissues typically in low numbers;

• highest numbers in skin, mucous membranes (respiratory, urogenital, and gut).

PMN

Polymorphonuclear cells→

• Neutrophils

• Eosinophils

• Basosinophils

What is the main function of neutrophils?

• Phagocytosis

• Intracellular degranulation

• Regulate inflammation

• NET formation

  • neutrophil extracellular traps

What are eosinophils involved in?

• Parasite defense and allergic responses

• Less efficient at phagocytosis

What is the main function of basophils?

Release histamines and heparin to control allergens

• Histamine causes allergic reactions

What are some major differences between innate and acquired immunity?

• Innate: fast, non-specific, no memory

• Acquired: slow, specific, has memory

What are the 3 different models of disease in populations?

• SI

• SIR

• SIRS

What is the SI model?

• Susceptible and infected

• Hard to recover from

• EX: Rabies – almost all infected mammals die

What is the SIR model?

• Animals will recover but will stay infected

• ex: many viral diseases in which immunity lasts for ~ the life of the host

What is the SIRS model?

• Susceptible → Infected → Resistant → Susceptible again

• ex: → many bacterial diseases in which hosts may become immune for a period of time, and before immunity wanes, they become susceptible again

What disease model fits lifelong immunity?

SIR model

What disease model fits temporary immunity?

SIRS model

Which Ig is which?

• 1→ IgG

• 2→ IgM

• 3→ IgD

• 4→ IgA

• 5→ IgE