Immune System

Infectious Agents

Bacteria

Single cell prokaryote
Ex: Clostridium tetani
Treatment: Antibiotics (watch for resistance)

Virus

Pieces of RNA or DNA
Ex: Chickenpox
Treatment: Antivirals or vaccines

Fungus/Parasite

Fungus / Parasite

Eukaryotic cells with membrane and wall
Ex: Yeasts
Opportunistic infections

Q:What is the difference between viral and bacterial infections?

Bacteria can reproduce inside or outside of the body, and does not hijack the cell to replicate. Most bacteria is good, however some can cause infection. Viruses are bits of RNA or DNA encapsulated by protein and must take over the hosts cell in order to reproduce and infect the body.

Immune cells

Dendritic Cell

Location: Skin and mucosal membranes
Function: Capture, process, and present antigens to T-cells
Key antigen-presenting cell

Mast Cell

Location: Connective tissue
Function: Fight infections by producing histamines which can cause inflammation to draw in other WBCs
Involved in allergic reactions

Macrophage

Location: Secondary lymphoid structures
Function: Phagocytose and destroy pathogens
Engulfs pathogens and debris

Microglia

Location: Brain and spinal cord
Function: Phagocytose pathogens in the CNS. Activate and release immune factors such as cytokines and chemokines.
Immune defense of brain tissue

Complement Activation Pathways

A. Classical Pathway — triggered by antibody-antigen complex
B. Alternative Pathway — triggered by pathogens or C3 hydrolysis

➡ C3 Activation → C3b → [Fill in effects below]

Three Main Effects:

Opsonization: Complement protein binds to pathogens.
Cytolysis: Complement triggers splitting of target cell
Inflammation: Enhanced by complement

Fever & Inflammation

Fever

Caused by release of pyrogens acting on the hypothalamus.
Function of fever: Inhibits reproduction of pathogens, increase adaptive immunity, accelerates tissue repair, promotes interferon activity

Inflammation

The 5 cardinal signs are: 1. Redness 2. Swelling 3. Heat 4. Pain 5. Loss of function
Clinical Note
Sustained high fever (>103°F) can lead to complications like seizure or organ damage

Antigens, Antibodies, MHC I, MHC II

Antigen

A molecule that triggers an immune response.
What the antibody binds to

Antibody

A Y-shaped protein that binds to a specific antigen.
What binds to the antigen

MHC I

Displays endogenous antigens to CD 8 (cytotoxic) T cells.
Involved in CD 8 cell activation
Found on all nucleated cells
Ex: Viral infections, cancer

MHC II

Displays exogenous antigens to CD 4 (helper) T cells.
Involved in CD4 cell activation
Found on APCs
Ex: Bacterial infections

Antigen-Presenting Cells (APCs)

APC Type Function (Fill In) Location

Macrophage Phagocytose pathogens and serve as APCs. Tissues

Dendritic Cell Engulf cells and serve as APCs Skin & mucosa

B Cell To produce antibodies Blood & lymph

Helper (CD4) T Cell Activation

Antigen-presenting cell (exogenous antigen)

↓

Antigen displayed on MHC class II molecule

↓

Helper T cell receptor binds to antigen-antibody complex

↓

Co-stimulation via CD28 on T cell with CD80 or CD86 on APC’s

↓

Helper T cell activates and releases IL-2

↓

B cell and cytotoxic T cell activation

Cytotoxic (CD8) T Cell Activation

Infected cell presents antigen on MHC class I

↓

Cytotoxic T cell binds via its receptor

↓

Activation triggers release of perforin and granzymes

↓

Target cell lysis and apoptosis

Positive and Negative Selection

Where: Thymus
Goal: Ensure T cells can recognize MHC but not react to self antigens.

T cell development → Positive Selection → [Recognizes MHC]

↓

Negative Selection → [Does not recognize self antigen]

Clinical Questions

How does understanding the immune system help you as a future nurse recognize early signs of infection?

Recognizing certain immune responses and symptoms of immune responses, as well as the role that certain lymph organs and WBCs play can all help in understanding and identifying certain types of infections, so we can know how to treat them.

Why must nurses understand the difference between bacterial and viral infections before administering antibiotics?

Because antibiotics only work on bacterial infections. Viral infections can be prevented through vaccines, however treatment is mostly symptom based and the viral infection has to run its course.

How do vaccinations make use of the immune system’s memory?

Vaccines contain weakened/dead microorganisms or components which stimulate B-lymphocyte memory cell production. This way, if the body comes into contact with a pathogen, it knows how to produce an immune response against it.