Positive Selection

Definition

Positive selection is a critical process during the development of immune cells, particularly T cells and B cells, where cells that successfully recognize self-MHC (Major Histocompatibility Complex) molecules are allowed to survive and mature, while those that do not are eliminated. This ensures that only immune cells capable of recognizing foreign antigens presented by MHC molecules remain functional, thereby contributing to effective immune responses.

5 Must Know Facts For Your Next Test

1. Positive selection occurs primarily in the thymus for T cells and in the bone marrow for B cells.
2. In T cell development, only thymocytes that can moderately bind to self-MHC molecules receive survival signals, allowing them to continue maturing.
3. The absence of positive selection leads to a lack of functional T cells, which can result in severe immunodeficiency.
4. For B cells, positive selection happens when immature B cells recognize self-antigens with low affinity; those that do not receive survival signals undergo apoptosis.
5. Positive selection is crucial for the generation of a diverse yet self-tolerant repertoire of immune cells capable of responding effectively to pathogens.

Review Questions

• How does positive selection contribute to the overall functionality of the immune system?
  • Positive selection helps ensure that only those T and B cells capable of recognizing foreign antigens presented by MHC molecules survive and mature. This process enables the immune system to have a diverse array of functional immune cells that can effectively respond to various pathogens while minimizing the risk of autoimmunity. The successful recognition and interaction between immune cells and self-MHC molecules is fundamental for mounting effective immune responses.
• Compare and contrast positive selection with negative selection in the context of T cell development.
  • Positive selection and negative selection are two essential processes in T cell development that ensure a competent yet tolerant immune response. While positive selection selects for T cells that can moderately bind to self-MHC molecules, allowing them to survive and mature, negative selection eliminates T cells that strongly bind to self-antigens to prevent autoimmunity. Together, these processes create a balanced repertoire of T cells capable of recognizing pathogens without attacking the body’s own tissues.
• Evaluate the consequences of defective positive selection on the immune system's ability to fight infections.
  • Defective positive selection can lead to insufficient numbers of functional T and B cells in the immune system, which compromises its ability to respond effectively to infections. Without a robust population of mature immune cells capable of recognizing foreign antigens, the body becomes more susceptible to various pathogens. This can result in recurrent infections or an inability to mount appropriate adaptive immune responses, leading to greater health risks and potential autoimmune disorders if some self-reactive cells escape negative selection.

Related terms

Negative Selection: The process that eliminates immune cells that strongly bind to self-antigens, preventing autoimmune reactions and ensuring self-tolerance.

Thymus: An organ where T cell maturation occurs, playing a vital role in both positive and negative selection of T cells.

B Cell Receptor (BCR): A membrane-bound immunoglobulin on B cells that allows them to recognize specific antigens, essential for positive selection in B cell development.