The Immune System

What constitutes the First Line of Defense, and is it specific or non-specific?

• Components: External physical barriers like the skin and mucous membranes (e.g., nose lining).

• Type: Non-specific (innate) defense because it indiscriminately blocks any potential invader from entering the body.

What role do mast cells and histamine play in the Second Line of Defense (inflammatory response)?

• When physical barriers are breached, mast cells release histamine.

• Histamine causes nearby blood vessels to dilate (widen) and become more permeable (leaky).

• This allows white blood cells (like macrophages) to infiltrate the tissue, engulf, and digest pathogens.

What is the Complement System and how does it support immunity?

A system that enhances immune functions in both non-specific and specific responses. The release of complement factors helps draw more macrophages to the site of infection to destroy pathogens.

What is an antigen and how does Adaptive Immunity differ from innate immunity?

• Antigen: A substance recognized by the body as foreign (often pieces/proteins of pathogens).

• Adaptive Immunity: The third line of defense; it is a targeted, specific response activated when the first two innate lines of defense fail to control the pathogen.

How do Cytotoxic T-Cells destroy infected cells in Cell-Mediated Immunity?

• They recognize pathogen antigens presented on compromised cell surfaces.

• They release perforin, which creates holes in the infected cell’s membrane, causing it to take in water/ions and burst.

• This induces apoptosis (programmed cell death) to stop pathogen replication.

What is the primary role of Helper T-Cells?

They act as central coordinators for both cell-mediated and humoral responses. When macrophages present antigens to Helper T-cells, the Helper T-cells release chemical signals that stimulate both cytotoxic T-cells and B-cells.

What is Humoral Immunity and what cells produce antibodies?

• Humoral immunity is antibody-mediated defense.

• Activated B-cells differentiated and produce antibodies that circulate in blood, lymph, saliva, mucus, and breast milk to neutralize antigens and mark them for destruction by macrophages.

What are the distinct roles and locations of the 5 antibodies classes?

• IgG: Most abundant antibody, found in body fluids; protects against bacterial/viral infections, neutralizes toxins, and triggers complement system.

• IgA: Found in mucous membranes (GI and respiratory tracts), saliva, tears, and breast milk; critical for mucosal immunity.

• IgM: First antibody produced in response to an infection; present in blood and lymph.

• IgE: Primarily associated with allergic reactions and defense against parasitic infections; found in skin, lungs, and mucous membranes.

• IgD: Mainly located on the surface of immature B-lymphocytes; plays a role in early immune activation.

What is the function of Memory B-cells vs. Memory T-cells?

• Both retain a memory of encountered antigens for faster future responses.

• Memory B-cells: Rapidly produce plasma B-cells to generate antibodies upon re-exposure.

• Memory T-cells: Rapidly activate cytotoxic T-cells to destroy infected cells upon re-exposure.

What is Active Immunity, how is it acquired, and how long does it last?

• Mechanism: The body’s own immune system produces antibodies and memory cells in response to an antigen.

• Acquired by: Direct natural exposure (catching a cold) or artificial vaccination (weakened/dead pathogen).

• Duration: Long-lasting/often lifelong.

What is Passive Immunity, how is it acquired, and how long does it last?

• Mechanism: A person is given pre-formed antibodies from another source (the body makes no memory cells).

• Acquired by: Natural transfer via placenta/breast milk or artificial antibody serum (e.g., rabies treatment or anti-venom).

• Duration: Immediate, but temporary (last a few weeks to months before antibodies degrade).