Full Answer Section

• Effector Actions:
  • Phagocytosis: Specialized cells (e.g., neutrophils, macrophages) engulf and destroy microbes or cellular debris.  
  • Inflammation: A complex response involving the recruitment of immune cells and fluid to the site of infection or injury, leading to redness, swelling, heat, and pain.  
  • Cytokine Release: Immune cells release cytokines (signaling molecules) that coordinate the immune response, activating other cells and influencing their behavior.  

b. Describe granulocytes, which type of them is the most abundant and why?

• Granulocytes: A type of white blood cell characterized by the presence of granules in their cytoplasm. These granules contain enzymes and other substances that are released during an immune response.  
• Types:
  • Neutrophils: Most abundant.  
  • Eosinophils:
  • Basophils:
• Most Abundant and Why: Neutrophils are the most abundant type of granulocyte (and white blood cell overall), typically making up 50-70% of circulating white blood cells. This is because they are the first responders to bacterial infections and are highly efficient at phagocytosis. They are short lived, and constantly being produced, so there are always many present.  

c. Why can bone marrow transplants be used to replace defective lymphocytes?

• Hematopoietic Stem Cells: Bone marrow contains hematopoietic stem cells, which are the precursors to all blood cells, including lymphocytes (T cells and B cells).  
• Differentiation: These stem cells can differentiate into all the different types of immune cells.
• Replacement: By transplanting healthy bone marrow, the defective lymphocytes can be replaced with new, functional lymphocytes produced by the transplanted stem cells.

d. What is inflammatory response; describe the two general events that initiate the inflammation and two changes in cells undergoing apoptosis.

• Inflammatory Response: A complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. It’s a protective mechanism that helps eliminate the initial cause of cell injury, clear out necrotic cells and tissues damaged from the original insult and the inflammatory process, and initiate tissue repair.  
• Initiating Events:
  • Tissue Damage: Physical injury, chemical irritation, or other forms of tissue damage release DAMPs, which trigger inflammation.  
  • Pathogen Recognition: PRRs on immune cells recognize PAMPs from invading microbes, initiating an inflammatory response.  
• Changes in Cells Undergoing Apoptosis:
  • Cell Shrinkage: The cell shrinks in volume.
  • Blebbing: The plasma membrane forms bubble-like protrusions called blebs.
  • Nuclear Fragmentation: The nucleus breaks down into fragments.
  • Formation of Apoptotic Bodies: The cell is broken into small membrane bound packages, that are then phagocytosed.  

2. Applications: Patients who had recently a bone marrow transplant are very susceptible to infection. Why? Explain!  

• Immunosuppression: To prevent rejection of the transplanted bone marrow, patients receive immunosuppressant drugs, which suppress the immune system.  
• Immature Immune System: The new immune system takes time to develop and become fully functional. During this period, the patient has a weakened immune response and is highly vulnerable to opportunistic infections.  
• Lack of Prior Immunity: The patient’s new immune system lacks prior exposure to many pathogens, making them susceptible to common infections.

II. Adaptive Immune System

1. MicroAssessment

a. How is the cell-mediated response different from the humoral one?

• Cell-Mediated Response:
  • Involves T lymphocytes (specifically cytotoxic T cells).
  • Targets intracellular pathogens (e.g., viruses, bacteria inside cells) and abnormal cells (e.g., cancer cells).
  • Works by directly killing infected or abnormal cells.
• Humoral Response:
  • Involves B lymphocytes and antibodies.  
  • Targets extracellular pathogens (e.g., bacteria, toxins in body fluids).
  • Works by producing antibodies that neutralize pathogens, opsonize them for phagocytosis, or activate the complement system.  

b. Why would a person with AIDS be more susceptible to bacteria that cause tuberculosis?

• CD4+ T Cell Depletion: AIDS is caused by HIV, which targets and destroys CD4+ T cells (helper T cells).  
• Weakened Cell-Mediated Immunity: CD4+ T cells are crucial for coordinating the immune response, including the activation of macrophages, which are essential for controlling intracellular pathogens like Mycobacterium tuberculosis (the bacterium that causes tuberculosis).  
• Increased Susceptibility: With a depleted CD4+ T cell count, the cell-mediated immune response is severely weakened, making individuals with AIDS highly susceptible to tuberculosis and other opportunistic infections.

c. What mechanism do NK cells use to kill “self” cells?

• Lack of MHC Class I: NK cells recognize and kill cells that lack or have reduced expression of MHC class I molecules on their surface.  
• Perforin and Granzymes: When an NK cell encounters a target cell lacking MHC class I, it releases perforin and granzymes. Perforin creates pores in the target cell membrane, and granzymes enter the cell, triggering apoptosis (programmed cell death).  

2. Applications: What kind of diseases would be expected to happen (occur) as a result of lack of T and B lymphocytes?

• Severe Combined Immunodeficiency (SCID): This is a primary immunodeficiency disease characterized by the absence or severe deficiency of both T and B lymphocytes.  
• Increased Susceptibility to Infections: Individuals with SCID are highly susceptible to a wide range of infections, including bacterial, viral, fungal, and protozoal infections.  
• Opportunistic Infections: They are particularly vulnerable to opportunistic infections, which are caused by pathogens that typically do not cause disease in healthy individuals.  
• Failure to Thrive: Children with SCID often fail to thrive due to recurrent infections and impaired immune function.  
• Increased Risk of Cancer: A lack of T cells, which are important in killing tumor cells, increases the risk of certain cancers.
• Autoimmune Diseases: In some cases, a lack of proper T cell regulation can lead to autoimmune diseases, where the immune system attacks the body’s own tissues.