The Immune System: Nonspecific and Specific Defenses

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Immune System

Overview

The immune system is responsible for defending the body against invading pathogens and abnormal cells. It is divided into two main types of defenses: nonspecific (innate) defenses and specific (adaptive) defenses.

Nonspecific Defenses: Provide general protection against a wide range of pathogens.
Specific Defenses: Target particular pathogens with a tailored response and memory.

Nonspecific Defenses

Types of Nonspecific Defenses

Physical barriers
Phagocytes
Immunological surveillance
Interferons
Complement system
Fever
Inflammation

Physical Barriers

Epithelial coverings: Skin and mucosal linings act as the first line of defense, preventing pathogen entry.
Secretions: Mucus, wax, saliva, and acids trap or destroy pathogens.
Hair and nails: Provide additional mechanical protection.

Phagocytes

Microphages: Include neutrophils and eosinophils; circulate in blood and quickly respond to infection.
Macrophages: Found in most tissues; engulf and destroy or mark invading pathogens for destruction.

Types of Macrophages

Fixed macrophages: Reside in specific tissues (dermis, bone marrow, brain—microglia, liver, lungs, lymph nodes, spleen).
Free macrophages: Migrate through tissues, attracted or repelled by chemicals (chemotaxis), and can squeeze between capillary cells (diapedesis); derived from monocytes.

Immunological Surveillance

Natural Killer (NK) Cells: A type of lymphocyte (5–10% of lymphocytes) that recognizes and destroys abnormal cells, including bacteria, virus-infected cells, and cancer cells.

Action of NK Cells

Respond to abnormal membrane proteins (antigens).
Golgi apparatus points toward the abnormal cell.
Release perforins that break down the membrane of the abnormal cell, causing cytolysis.

Interferons

Small proteins released by lymphocytes, macrophages, and virus-infected cells.
Block viral replication in neighboring cells, limiting the spread of infection.

Complement System

Composed of more than 20 plasma proteins that interact in a cascade.
Activated by either the classic (antibody-antigen complex) or alternate (antigen-triggered) pathway.

Effects of Complement Activation

Destruction of invader cell membranes (membrane attack complex creates holes).
Promotes inflammation.
Attracts phagocytes (chemotaxis).
Opsonization: Coats antigens to enhance phagocyte binding.

Fever (Pyrexia)

Inhibits some pathogens.
Speeds up metabolism, enhancing body defenses.

Cause of Fever

Triggered by pyrogens (chemicals that reset the hypothalamic thermostat), including pathogens, bacterial toxins, antigen-antibody complexes, and chemicals released by macrophages.

Inflammation

Response to tissue injury or chemical irritation.
Produces swelling, heat, redness, pain, and loss of function.
Goals: Speed repair, slow pathogen spread (clotting), and recruit other immune cells (chemotaxis).

Steps in the Inflammatory Response

Mast cells (basophils) release histamine, heparin, and prostaglandins, increasing blood flow and initiating clotting.
Increased blood flow brings phagocytes and removes toxins.
Elevated temperature speeds reactions and denatures foreign proteins.
Complement activation enhances the response.

Specific Defenses (Adaptive Immunity)

Overview

Specific defenses involve the activities of T and B lymphocytes and confer immunity by recognizing and responding to specific antigens.

Immunity

Ability of the body to defend itself against invading agents (antigens).
Distinguishes self-antigens from non-self antigens and neutralizes non-self threats.

Lack of Resistance

Vulnerability (age-related)
Susceptibility (breach in immune system)
Immunocompromised, immunosuppressed (iatrogenic), immunodeficient (e.g., HIV)

Antigens (Antibody Generators)

Molecules that trigger an immune reaction (e.g., glycoproteins, proteins, and other large molecules).
Found on cell membranes and walls of microorganisms and foreign cells.

Characteristics of Immunity

Specificity: Targets particular antigens.
Antibodies: Produced specifically for each antigen.
Memory: Remembers previous encounters for faster, stronger responses.

Memory Cells and Antibodies

Memory cells: Copies of specific T and B lymphocytes that remain after an infection, allowing rapid response to future invasions by the same organism.
Antibodies: Large protein molecules that bind to and destroy specific antigens; remain in the system after infection for future defense.

Types of Immunity

Type	How Acquired
Innate	Genetic, present at birth
Acquired	Developed after exposure
— Naturally acquired active	Exposure to pathogen (antigen)
— Natural passive	Antibodies from mother to fetus/infant
— Artificial/Induced active	Immunization (vaccines)
— Artificial/Induced passive	Antibodies administered to fight infection

Cells Involved in Immunity

T Cells (80% of Lymphocytes)

Cytotoxic (Killer) T Cells (CK): Directly attack antigens (80–85% of T cells).
Helper T Cells (T4): Activate B cells and other T cells.
Suppressor T Cells: Inhibit B and T cell activity.
Memory T Cells: React to previously encountered antigens, becoming CK or helper T cells upon re-exposure.

B Cells (Bone Marrow, 10–15% of Lymphocytes)

Form plasma cells that produce antibodies (immunoglobulins) and bind to antigens.
Responsible for humoral immunity and creation of memory B cells.

Types of Antigens—Self Antigens

Type	Location	Function
MHC Class I	All body cells	Normal in healthy cells; abnormal in damaged or viral-infected cells
MHC Class II	Antigen-presenting cells (monocytes, macrophages, B lymphocytes)	Present antigens to T cells

Types of Immune Responses

Cell-mediated immune responses: Activation of cytotoxic T cells to directly attack invaders.
Antibody-mediated immune responses: Activation of B cells to produce antibodies specific for invaders.

Cell-Mediated Immunity

Begins with activation of T cells by an antigen.
Leads to production of several types of T cells specific to the invader and destruction of the invader.

T Cell Antigen Receptors (TCRs)

Recognize foreign antigens presented with MHC proteins.
Each T cell has unique TCRs; most are usually inactive until activated.

T Cell Activation

Recognition and binding to a matching antigen.
Costimulation (additional signals required for full activation).

T Cell Recognition of Antigens

T cells only recognize antigens presented by other cells (antigen-presenting cells or infected/damaged body cells).
Antigen is inserted into an MHC complex and displayed on the cell surface.

Antibody-Mediated Immunity

Begins with activation of B cells (in spleen, lymph nodes, and lymph nodules).
Results in production of plasma cells (secrete antibodies) and memory cells.

Classification of Antibodies (Immunoglobulins)

Type	Main Function
IgG	Most common; crosses placenta
IgA	Secretory (found in mucus, saliva, etc.)
IgM	Blood group antibodies
IgE	On basophils; involved in allergic responses
IgD	Function not fully understood; involved in B cell activation

Antigen-Antibody Complex Effects

Block antigen binding sites.
Agglutination (clumping of antigens).
Activation of complement system.
Attraction of phagocytes (chemotaxis).
Opsonization (coating to enhance phagocytosis).
Promote inflammation.
Block antigen invasion (coat epithelium).

Patterns of Immune Response

Early stages of infection: Neutrophils and NK cells attack bacteria.
Later stages: Cytokines draw in macrophages; cytotoxic T cells arrive.
Final stages: B cells activate and plasma cells increase antibody production.

Additional info: The immune system's ability to distinguish self from non-self is critical for preventing autoimmune diseases. Immunological memory is the basis for the effectiveness of vaccines.