[Imunlogia] 6 - Anticorpos e antígenos

Introduction to Immunology: Antigens and Antibodies

Overview of Antibodies

• Antibodies are proteins circulating in plasma, serving as key mediators of humoral immunity.

• Each antibody is specific to a particular antigen, necessitating a diverse array of antibodies in the bloodstream to combat various antigens.

• Antibodies can exist freely or be bound to B lymphocyte membranes, acting as receptors for specific antigens.

Functions of Antibodies

• The primary function of an antibody is to activate B cells upon binding with their specific antigen, leading to the production and secretion of antibodies against that antigen.

• Once secreted, antibodies trigger effector mechanisms in humoral immunity; they primarily bind specifically to their target antigens.

Mechanisms of Action

• One action is neutralization, where antibodies block microorganisms or toxins from interacting with host cells by forming a mechanical barrier.

• This neutralization prevents pathogens from attaching to cell surfaces and inhibits toxic effects from substances like toxins.

Opsonization and Phagocytosis

• Another critical role is opsonization, where antibodies mark antigens for recognition by immune cells such as macrophages and neutrophils.

• Opsonized antigens facilitate phagocytosis due to receptors on immune cells recognizing the bound antibodies.

Activation of Complement System

• Antibodies also activate the complement system through classical pathways when they bind with antigens, enhancing immune responses.

• Activated complement components aid in destroying pathogens by forming membrane attack complexes that disrupt microbial membranes.

Structure and Classes of Antibodies

Biochemical Structure

• An antibody consists of four chains linked by disulfide bonds: two heavy chains (identical pairs), and two light chains (also identical).

• Each antibody has two identical antigen-binding sites formed by portions of both heavy and light chains known as variable regions.

Constant vs. Variable Regions

• The constant region remains unchanged across different antibodies within a class while variable regions differ among classes allowing specificity for various antigens.

Types of Heavy Chains

• There are five types of heavy chains (alpha, delta, epsilon, gamma, mu), determining the class (IgA, IgD, IgE, IgG, IgM).

Functional Diversity Among Classes

Class-Specific Functions

• Each immunoglobulin class has distinct functions based on its heavy chain type; for example:

• IgA protects mucosal surfaces,

• IgM is the first responder during infections,

• IgG crosses the placenta providing fetal immunity.

Light Chains and Their Role

Types of Light Chains

• There are two types: kappa (κ) and lambda (λ); each antibody contains only one type but can have either form.

Understanding Antigens

Definition and Epitope Recognition

• An antigen induces an immune response; however, only specific parts called epitopes are recognized by antibodies.

Immunogenicity vs. Tolerance

• Immunogens elicit an immune response,

• Tolerogens do not provoke any response despite being recognized.

Changes Post-Antigen Encounter

Affinity Maturation

• Upon encountering an antigen via B cell receptors:

• The affinity between antibody and antigen increases over time through affinity maturation processes.

Secretion Transition

• Initially membrane-bound antibodies transition into secreted forms after activation leading to free circulation in blood plasma.

Class Switching

• Following activation signals from T helper cells:

• B cells may switch from producing one class (e.g., IgM initially produced during infection), transitioning into other classes based on cytokine influence.

Conclusion

Summary Remarks

• The session concludes with encouragement for further engagement through comments or subscriptions for ongoing learning about immunology topics.