Immune System Overview

Definition and Function

The immune system is the body's defense mechanism against pathogens and harmful agents. It distinguishes between self (the body's own cells) and non-self (foreign invaders such as bacteria, viruses, parasites, and allergens).

• Self: Includes normal and defective cells of the body.

• Non-self: Includes bacteria, viruses, parasites, allergens, and other pathogens.

The immune system operates through a series of barriers and responses to protect the body from infection and disease.

Lines of Defense

• First Line of Defense: Physical, chemical, and mechanical barriers (e.g., skin, mucous membranes, stomach acid).

• Second Line of Defense: If pathogens bypass barriers, the immune system responds in four steps:

  1. Detection & identification of the pathogen

  2. Communication with other immune cells

  3. Recruitment of assistance and coordination of response

  4. Destruction or suppression of the pathogen

Innate and Adaptive Immunity

Innate Immune Response

The innate immune response is rapid, non-specific, and does not generate memory. It includes barriers and cellular responses such as phagocytosis and inflammation.

• Physical barriers: Skin, mucous membranes

• Chemical barriers: Stomach acid, enzymes in saliva

• Cellular responses: Phagocytes (macrophages, neutrophils), natural killer cells

Adaptive Immune Response

The adaptive immune response is slower, highly specific, and generates immunological memory. It involves lymphocytes (B cells and T cells) and the production of antibodies.

• Cell-mediated immunity: T lymphocytes attack infected or abnormal cells.

• Antibody-mediated immunity: B lymphocytes produce antibodies that neutralize pathogens.

Pathogens of the Human Body

Bacteria vs. Viruses

Bacteria and viruses are the main types of pathogens, each requiring different defense mechanisms due to differences in structure, reproduction, and susceptibility to drugs.

• Bacteria: Living cells, can survive outside host, susceptible to antibiotics.

• Viruses: Non-living, require host cells to replicate, susceptible to antiviral drugs.

Table: Differences between Bacteria and Viruses

Immune Mechanisms

Membrane Attack Complex

The membrane attack complex (MAC) is formed by complement proteins that insert into the membrane of a pathogen, creating pores. This allows water and ions to enter, causing the cell to swell and lyse.

• Complement proteins assemble on pathogen membrane

• Pores allow influx of water and ions

• Pathogen cell swells and bursts (lysis)

Phagocytosis

Phagocytosis is the process by which immune cells (phagocytes) engulf and digest pathogens.

• Phagocyte recognizes and binds pathogen

• Pathogen is engulfed into a phagosome

• Lysosomes fuse with phagosome, releasing enzymes

• Pathogen is digested and antigen fragments are produced

Antigen-Presenting Cells (APCs)

Antigen-presenting cells (such as macrophages) display fragments of digested pathogens on their surface using MHC-II molecules. This presentation is crucial for activating T cells and initiating adaptive immunity.

• APCs process and present antigens on MHC-II

• Activates helper T cells

Antibodies and Their Functions

Antibody Structure

Antibodies are Y-shaped proteins produced by B lymphocytes. Each antibody consists of two identical light chains and two identical heavy chains, with antigen-binding sites at the tips of the arms.

• Variable region binds specific antigens

• Constant region determines antibody class

Antibody Functions

• Clumping (agglutination) of antigens

• Neutralization of bacterial toxins

• Act as opsonins to enhance phagocytosis

• Trigger degranulation of immune cells

• Activate complement proteins

• Antibody-dependent cell-mediated cytotoxicity

• Activate B lymphocytes

Lymphocytes and Immune Responses

Lymphocyte Clones

A clone is a group of lymphocytes specific to one antigen. Each clone responds to a unique antigen and proliferates upon activation.

T Lymphocytes

• Cytotoxic T cells: Destroy cells displaying foreign antigens on MHC-I by releasing perforins and granzymes, or activating Fas for apoptosis.

• Helper T cells (TH): Bind to immune cells displaying foreign antigen in MHC-II, secrete cytokines to activate other immune cells. HIV targets these cells.

• Regulatory T cells (Tregs): Suppress immune cell function to prevent excessive response; bind to MHC-II complexes.

Primary and Secondary Immune Responses

• Primary response: First exposure to antigen; slower, lower antibody production.

• Secondary response: Subsequent exposures; faster and stronger due to memory cells.

Types of Immunity

Active vs. Passive Immunity

• Active immunity: Body produces its own antibodies after exposure to antigen.

  • Natural: Infection

  • Artificial: Vaccination

• Passive immunity: Antibodies are acquired from another organism.

  • Natural: Through placenta or breast milk

  • Artificial: Injection of gamma globulins

Recognition of Foreign Tissue

MHC Proteins and Blood Groups

Major Histocompatibility Complex (MHC) proteins, also known as human leukocyte antigens (HLA), allow immune cells to recognize self and non-self. Red blood cells lack MHC proteins but have specific antigens that determine blood type.

• Four main blood types: A, B, AB, O

• Individuals express antibodies against RBC antigens they do not possess

• Transfusion mismatch can cause RBC lysis via complement cascade

• Rh factor: Hemolytic disease of the newborn can occur if mother is Rh- and fetus is Rh+

Table: ABO Blood Groups

Example: A person with type A blood has A antigens on their RBCs and anti-B antibodies in their plasma. Receiving type B blood would trigger an immune response and cause hemolysis.

Additional info: The notes have been expanded with academic context and definitions for clarity and completeness.