Immunology Study Guide: T Cells, Immunity, Vaccines, and Hypersensitivity

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Immunology: T Cells and Immune Responses

T Cell Receptors and Activation

T cells are a critical component of the adaptive immune system, responsible for recognizing and responding to specific antigens. Their activation and function depend on several molecular signals and cellular interactions.

T Cell Receptor (TCR): The TCR is composed of variable and constant regions, with the variable region responsible for antigen binding.
Two Signals for Activation: T cells require two signals for activation: (1) specific antigen binding and (2) differentiation of pathogen from non-pathogen. This ensures T cells are only activated by appropriate antigens presented by phagocytes.
IL-2 Role: Interleukin-2 (IL-2) is crucial for T cell proliferation and auto-stimulation, tuning the strength of activating signals.
Signal Failure: T cells that do not receive both signals fail to activate and may undergo anergy or apoptosis.

Types of T Cells

T cells can be classified based on their function and activation status.

Naive Cells: These are T cells (and other immune cells like macrophages) that have not yet encountered their specific antigen.
Effector Cells: Activated T cells that perform immune functions, such as cytotoxicity or helping other immune cells.
Helper T Cells (Th): These cells assist other immune cells by releasing cytokines and enhancing immune responses.
Special Features: Activated macrophages have enhanced phagocytic and antigen-presenting capabilities.
Memory T Cells: Provide long-term immunity by "remembering" previous encounters with antigens.
Regulatory T Cells: Suppress immune responses to prevent autoimmunity.

Natural Killer (NK) Cells

NK cells are part of the innate immune system and are responsible for killing infected or abnormal cells.

Function: NK cells kill cells that lack normal MHC markers, such as virus-infected or tumor cells.
Comparison with T Cells: Unlike cytotoxic T cells, NK cells do not require antigen presentation for activation.
Selective Killing: NK cells do not kill all cells in the body; they target only those lacking self markers.

Immunity and Vaccines

Types of Immunity

Immunity can be classified as natural or artificial, and as active or passive.

Natural Immunity: Acquired through natural exposure to pathogens.
Artificial Immunity: Acquired through medical intervention, such as vaccination.
Active Immunity: The body produces its own antibodies after exposure to an antigen.
Passive Immunity: Antibodies are transferred from another source (e.g., maternal antibodies, antiserum).
Herd Immunity: When a large portion of the community is immune, providing indirect protection to non-immune individuals.

Vaccines

Vaccines are biological preparations that provide immunity against specific diseases.

Attenuated Vaccines: Contain live, weakened pathogens. Provide long-lasting immunity but may not be suitable for immunocompromised individuals.
Inactivated Vaccines: Contain killed pathogens. Safer but may require booster doses.
Types of Inactivated Vaccines: Toxoid, subunit (acellular), polysaccharide, and protein conjugate vaccines.
DNA/mRNA Vaccines: Induce immunity without exposure to the actual disease agent.
Oral Vaccines: Administered orally, such as the oral polio vaccine.
Advantages: Both inactivated and live vaccines have unique benefits regarding safety and duration of immunity.

Serological Tests and Antibody Detection

Serological tests are used to detect antibodies and diagnose diseases.

Precipitin Serological Test: Detects antigen-antibody complexes. Variants include immunodiffusion, immunoelectrophoresis, and agglutination.
ELISA (Enzyme-Linked Immunosorbent Assay): Used to detect and quantify antibodies. Can be direct or indirect.
Western Blot: Uses antibodies to detect specific proteins separated by electrophoresis.
Monoclonal vs. Polyclonal Antiserum: Monoclonal antibodies are specific to a single epitope, while polyclonal antiserum contains antibodies against multiple epitopes.

Hypersensitivity and Autoimmunity

Types of Hypersensitivity (HS) Reactions

Hypersensitivity reactions are exaggerated immune responses that can cause tissue damage.

Type I HS (Allergy): Immediate hypersensitivity mediated by IgE antibodies. Symptoms include allergic rhinitis, asthma, and anaphylaxis.
Antihistamines: Effective treatment for Type I HS by blocking histamine receptors.
Desensitization: Gradual exposure to allergens can reduce sensitivity.
Type II HS: Cytotoxic reactions involving non-native surface proteins (e.g., blood transfusion reactions, Rh incompatibility).
Type III HS: Immune complex-mediated reactions.
Type IV HS: Delayed-type hypersensitivity (e.g., contact dermatitis).
Comparison: Type II and Type IV HS differ in mechanism and clinical presentation.

Autoimmune Diseases

Autoimmune diseases occur when the immune system attacks the body's own tissues.

Examples: Myasthenia gravis, type I diabetes, rheumatoid arthritis, systemic lupus erythematosus.
Mechanisms: Loss of self-tolerance, genetic predisposition, and environmental triggers.
Treatment: Immunosuppressive drugs and therapies targeting specific immune pathways.

Immunodeficiency

Immunodeficiency refers to the inability of the immune system to mount an adequate response.

Inherited Immunodeficiency: Genetic disorders affecting immune cell development or function.
Acquired Immunodeficiency: Resulting from infections (e.g., HIV), malnutrition, or medical treatments.
Severity: Immunodeficiency disorders vary in frequency and severity, affecting susceptibility to infections and disease progression.

Table: Comparison of Types of Immunity

Type	Source	Duration	Example
Natural Active	Infection	Long-term	Recovery from measles
Natural Passive	Maternal antibodies	Short-term	Antibodies in breast milk
Artificial Active	Vaccination	Long-term	MMR vaccine
Artificial Passive	Antiserum	Short-term	Rabies immunoglobulin

Table: Types of Hypersensitivity Reactions

Type	Mechanism	Examples
I (Immediate)	IgE-mediated, mast cell degranulation	Allergic rhinitis, asthma, anaphylaxis
II (Cytotoxic)	Antibody-mediated cell destruction	Blood transfusion reaction, Rh incompatibility
III (Immune Complex)	Antigen-antibody complex deposition	Serum sickness, Arthus reaction
IV (Delayed)	T cell-mediated	Contact dermatitis, tuberculin reaction

Key Equations

Antibody-Antigen Binding:
ELISA Quantification:
Where: = absorbance, = molar absorptivity, = concentration, = path length.

Additional info: Some explanations and examples have been expanded for clarity and completeness.