Immunity Overview

Innate vs. Adaptive Immunity

The immune system is divided into two main branches: innate immunity and adaptive immunity. Innate immunity provides immediate, non-specific defense against pathogens, while adaptive immunity offers a specific, long-lasting response and memory against previously encountered antigens.

• Innate Immunity: First line of defense, includes physical barriers (skin, mucous membranes), phagocytic cells, natural killer cells, inflammation, and complement proteins.

• Adaptive Immunity: Involves lymphocytes (T cells and B cells), antibodies, and memory cells. Activated after exposure to specific antigens.

• Key Difference: Adaptive immunity is specific and has memory; innate immunity is non-specific and always ready.

Complement Pathways

Activation and Function

The complement system is a group of proteins that enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells. It is part of both innate and adaptive immunity and can be activated by three main pathways:

• Classical Pathway: Triggered by antibodies bound to antigens. Part of adaptive immunity.

• Lectin Pathway: Activated by lectin binding to pathogen surfaces. Part of innate immunity.

• Alternative Pathway: Activated directly by pathogen surfaces, independent of antibodies. Part of innate immunity.

All pathways lead to the formation of the membrane attack complex (MAC), which destroys pathogens by creating pores in their membranes.

Antigen Recognition and Immunogenicity

Antigens and Epitopes

An antigen is any substance that can trigger an immune response. The specific part of the antigen recognized by the immune system is called an epitope or antigenic determinant.

• Immunogenicity: The ability of an antigen to provoke an immune response.

• Haptens: Small molecules that are not immunogenic by themselves but become immunogenic when attached to larger carrier proteins.

• Antigen Processing: Antigens can be proteins, carbohydrates, or lipids from pathogens or abnormal cells.

Major Histocompatibility Complex (MHC)

The MHC is a set of cell surface proteins essential for the acquired immune system to recognize foreign molecules. It helps the body distinguish self from non-self.

• MHC Class I: Found on all nucleated cells; presents endogenous antigens to cytotoxic T cells (CD8+).

• MHC Class II: Found on antigen-presenting cells; presents exogenous antigens to helper T cells (CD4+).

Lymphocytes: T Cells and B Cells

T Cells

T cells are a type of lymphocyte involved in cell-mediated immunity. They mature in the thymus and are responsible for recognizing and destroying infected or abnormal cells.

• Cytotoxic T Cells (CD8+): Destroy infected cells by recognizing antigens presented by MHC I.

• Helper T Cells (CD4+): Activate other immune cells by recognizing antigens presented by MHC II.

• Memory T Cells: Provide long-term immunity by remembering past infections.

B Cells

B cells are responsible for humoral immunity. They mature in the bone marrow and produce antibodies that neutralize pathogens.

• Plasma Cells: Differentiated B cells that secrete large amounts of antibodies.

• Memory B Cells: Remain in the body to provide rapid response upon re-exposure to the same antigen.

Antibodies

Structure and Function

Antibodies (immunoglobulins) are proteins produced by B cells that specifically bind to antigens. They help neutralize pathogens and mark them for destruction by other immune cells.

• Types: IgG, IgM, IgA, IgE, IgD (each with specific roles in immunity).

• Function: Neutralization, opsonization, activation of complement, and agglutination.

Immune Response: Primary and Secondary

Phases of Immune Response

The immune response occurs in two main phases:

• Primary Response: First exposure to an antigen; slower and less robust.

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

Example: After recovering from a viral infection, memory cells allow the body to respond more quickly if exposed again.

Self-Tolerance and Autoimmunity

Mechanisms of Self-Tolerance

The immune system must distinguish between self and non-self to prevent attacking the body's own cells. This is achieved through self-tolerance mechanisms during lymphocyte maturation.

• Central Tolerance: Occurs in the thymus (T cells) and bone marrow (B cells); self-reactive cells are eliminated.

• Peripheral Tolerance: Occurs in peripheral tissues; additional mechanisms prevent autoimmunity.

Autoimmunity: Failure of self-tolerance can lead to autoimmune diseases, where the immune system attacks the body's own tissues.

Summary Table: Innate vs. Adaptive Immunity

Key Terms and Definitions

• Antigen: Substance that triggers an immune response.

• Epitope: Specific part of an antigen recognized by immune cells.

• Antibody: Protein produced by B cells that binds to antigens.

• Complement: Group of proteins that enhance immune responses.

• Lymphocyte: White blood cell involved in adaptive immunity (T cells and B cells).

• Phagocyte: Cell that engulfs and digests pathogens.

• Immunogenicity: Ability of a substance to provoke an immune response.

• Self-tolerance: Mechanism preventing immune response against the body's own cells.

Relevant Equations

• Antibody-Antigen Binding:

• Complement Activation (Simplified):

Additional info:

• Some context and terminology were inferred from standard immunology and anatomy & physiology textbooks to clarify fragmented or unclear sections.

• Examples and definitions were expanded for clarity and completeness.