Adaptive Immunity

Definition and Importance

Adaptive immunity is the body's ability to recognize and mount a defense against specific invaders. It is a complex system of responses that is crucial for both healthcare and biotechnology due to its specificity and memory.

• Specificity: The immune response targets specific pathogens.

• Inducibility: Immune cells are activated in response to particular antigens.

• Clonality: Activated immune cells proliferate to form identical clones.

• Unresponsiveness to Self: The system does not attack normal body cells.

• Memory: The immune system remembers pathogens and responds more rapidly upon re-exposure.

Example: After recovering from chickenpox, the immune system can quickly respond to future exposures to the virus.

Lymphocytes and Their Roles

B Lymphocytes (B Cells) and T Lymphocytes (T Cells)

Lymphocytes are specialized white blood cells central to adaptive immunity. They are distinguished by their surface proteins and maturation sites.

• B cells: Mature in the bone marrow; responsible for antibody-mediated (humoral) immunity.

• T cells: Mature in the thymus; responsible for cell-mediated immunity against intracellular pathogens.

Example: B cells produce antibodies that neutralize bacteria, while T cells destroy virus-infected cells.

Types of Adaptive Immune Responses

Humoral vs. Cell-Mediated Immunity

Adaptive immunity is divided into two main types based on the cells and mechanisms involved.

• Humoral (Antibody-Mediated) Immunity: B cells produce antibodies targeting extracellular pathogens.

• Cell-Mediated Immunity: T cells act against intracellular pathogens such as viruses and some bacteria.

Example: Antibodies neutralize toxins in the blood, while cytotoxic T cells kill infected host cells.

The Lymphatic System

Structure and Function

The lymphatic system is a network of vessels, cells, tissues, and organs that transport lymph and support immune function.

• Lymphatic vessels: Carry lymph from tissues to the circulatory system.

• Lymph: Fluid similar to plasma, derived from leaked blood vessel fluid.

• Primary lymphoid organs: Red bone marrow (lymphocyte production and B cell maturation), thymus (T cell maturation).

• Secondary lymphoid organs: Lymph nodes, spleen, tonsils, and mucosa-associated lymphatic tissue (MALT).

Example: Lymph nodes filter lymph and are sites where immune responses are initiated.

Antigens and Epitopes

Properties and Recognition

Antigens are molecules recognized as foreign and targeted by the immune system. They contain specific regions called epitopes.

• Antigen: Any molecule the body deems worthy of attack.

• Epitope: The three-dimensional region of an antigen recognized by immune cells.

• Best antigens: Large (5,000–100,000 daltons), complex proteins or glycoproteins.

Example: Bacterial cell wall proteins and viral capsid proteins are strong antigens.

Major Histocompatibility Complex (MHC)

Classes and Function

MHC molecules are glycoproteins on cell membranes that present antigenic peptides to T cells.

• MHC Class I: Present on all nucleated cells; display endogenous antigens.

• MHC Class II: Present on antigen-presenting cells (APCs); display exogenous antigens.

• Antigen presentation: Antigens bind in the groove of MHC molecules for recognition by T cells.

T Lymphocytes (T Cells)

Types and Functions

T cells are produced in the bone marrow and mature in the thymus. They circulate in lymph and blood, migrating to secondary lymphoid organs.

• Cytotoxic T cells (Tc, CD8+): Destroy infected or abnormal cells.

• Helper T cells (Th, CD4+): Type 1 assist cytotoxic T cells; type 2 assist B cells.

• Regulatory T cells (CD4+, CD25+): Suppress autoimmune responses.

Example: Cytotoxic T cells kill virus-infected cells using perforin and granzyme.

T Cell Receptors (TCRs) and Antigen Recognition

Specificity and Activation

TCRs are composed of two proteins forming a single antigen-binding site. They recognize antigens only when presented by MHC molecules.

• TCRs: Bind to epitopes associated with MHC proteins, not free antigens.

• Activation: Requires antigen presentation by APCs.

Example: A dendritic cell presents a viral peptide via MHC II to a helper T cell.

Clonal Deletion and Self-Tolerance

Mechanism

Clonal deletion ensures that lymphocytes reactive to self-antigens are eliminated, preventing autoimmune disease.

• Process: Lymphocytes that bind to autoantigens undergo apoptosis.

• Importance: Maintains self-tolerance and immune system specificity.

Example: T cells that react to self-proteins in the thymus are deleted.

Cell-Mediated Immune Responses

Activation and Function

Cell-mediated immunity targets intracellular pathogens and abnormal cells through the activation and proliferation of T cells.

• Steps: Antigen presentation, helper T cell differentiation, clonal expansion, self-stimulation.

• Effector mechanisms: Cytotoxic T cells release perforin and granzyme to induce apoptosis in target cells.

• Memory T cells: Persist in lymphoid tissue and respond rapidly upon re-exposure to the antigen.

Example: Cytotoxic T cells kill cancer cells and virus-infected cells.

B Lymphocytes (B Cells) and Antibodies

Structure and Function

B cells mature in the bone marrow and are responsible for producing antibodies. Each B cell has identical B cell receptors (BCRs) specific to one epitope.

• Plasma cells: Activated B cells that secrete antibodies.

• Memory B cells: Long-lived cells that initiate rapid antibody production upon re-exposure.

Example: After vaccination, memory B cells enable a faster response to the pathogen.

Antibody Classes and Functions

Types and Mechanisms

Antibodies (immunoglobulins) are secreted proteins that bind antigens and mediate various immune functions.

• IgM: First antibody produced; effective in agglutination.

• IgG: Most common and long-lasting; provides majority of antibody-based immunity.

• IgA: Found in secretions (saliva, tears, mucus).

• IgE: Involved in allergic and antiparasitic responses.

• IgD: Function not well understood.

Antibody Functions:

• Activation of complement and inflammation

• Neutralization of toxins and pathogens

• Opsonization (enhancing phagocytosis)

• Killing by oxidation (production of reactive oxygen species)

• Agglutination (clumping of pathogens)

• Antibody-dependent cellular cytotoxicity (ADCC)

Example: IgG antibodies neutralize viruses and facilitate their clearance by phagocytes.

Coordination of Immune Responses: Cytokines

Types and Roles

Cytokines are soluble regulatory proteins that mediate communication between immune cells.

• Interleukins (ILs): Signal among leukocytes.

• Interferons (IFNs): Antiviral proteins and immune modulators.

• Growth factors: Stimulate stem cell division.

• Tumor necrosis factors (TNFs): Regulate immune responses and inflammation.

• Chemokines: Direct cell movement (chemotaxis).

Example: IL-2 stimulates T cell proliferation during an immune response.

Types of Acquired Immunity

Natural and Artificial, Active and Passive

Immunity can be acquired through natural exposure or artificial means (vaccination), and can be active (body produces its own response) or passive (antibodies are received from another source).

• Naturally acquired active immunity: Response to infection.

• Naturally acquired passive immunity: Maternal antibodies transferred to newborn.

• Artificially acquired active immunity: Vaccination.

• Artificially acquired passive immunity: Injection of antibodies (e.g., antivenom).

Example: Vaccines stimulate artificially acquired active immunity.

Additional info: Some details were expanded for clarity and completeness, including the mechanisms of antibody function and the process of clonal deletion.