Clonal Selection Theory in Immunology

Clonal Selection Theory

• The population of cells from the blast cell, all with the same specificity, is called a clone.
• The foreign antigen selects a lymphocyte with complementary receptors.
• There are approximately 1 x 10^12 different antibody receptors, which means 1 x 10^12 different B cell clones.

Clonal Selection

• There are many lymphocyte populations in the body, each recognizing a single antigen.
• When an antigen enters the body, it selects a specific clone and activates it.
• The small clone proliferates and differentiates to produce a much larger clone of antibody-secreting plasma cells and long-lived memory cells.
• All cells from this large clone are specific for the same antigen.

Antibody Formation & Memory

• Antibody-secreting plasma cells synthesize and secrete antibodies for several days and then die.
• Each plasma cell can synthesize 10 million antibody molecules per hour.
• Antibodies are secreted into the blood and other fluids, binding to specific antigens to prepare them for destruction.
• Long-lived memory cells react with the same antigen if it enters the body again at a later time.

Primary Immune Response

• The response of the acquired immune system the first time it is exposed to a particular antigen.
• Immediately after antigen challenge, there is a lag period of 5-10 days with no observable increase in specific serum antibodies.
• The primary immune response peaks within several weeks and then declines over the next few weeks to months.

Secondary Immune Response

• The response is faster, stronger, and longer-lasting when a person is exposed to the same antigen a second time.
• The immune system was primed to the antigen in the primary response, resulting in increased numbers of specific memory lymphocytes.
• Memory cells provide immunological memory and are responsible for the powerful secondary response.

Antibody Structure

• Antibodies are also called immunoglobulins and are proteins found in blood and other body fluids.
• The molecule has a Y-shaped structure, which can change to a T-shape because of a flexible hinge region.
• The molecule has two 'arms' (antigen-binding fragments, Fab) with high specificity for antigen-binding.

Antibody Classes

• There are five immunoglobulin (Ig) classes: IgD, IgM, IgG, IgA, and IgE.
• Each class of antibody has a different biological function.
• IgM is the first type of antibody produced after primary antigenic stimulation and lasts for weeks to months.
• IgG is the most abundant, persists for months to years after antigenic stimulation, and provides long-term protection.
• IgA is found in mucus covering mucosal surfaces and has a secretory piece.
• IgE is involved in immunity to worm infections of the intestinal tract and allergic reactions.

Biological Properties of Human Immunoglobulins

• IgG binds to antigen, crosses the placenta, and triggers the complement system.
• IgM binds to antigen and triggers the complement system.
• IgA binds to antigen and has a secretory piece.
• IgE binds to antigen and is involved in allergic reactions.
• IgD binds to antigen and is present on the surface of B lymphocytes.

Understanding the concept of clonal selection theory, where a population of cells with the same specificity is formed, and how it relates to the immune system and antibody receptors.