CD21

Definition

CD21, also known as complement receptor 2 (CR2), is a protein expressed on the surface of B cells and follicular dendritic cells that plays a vital role in the immune response by binding to complement components and facilitating B cell activation and differentiation. It acts as a bridge between the adaptive and innate immune systems, enhancing the ability of B cells to respond to antigens that are opsonized with complement proteins.

5 Must Know Facts For Your Next Test

1. CD21 is essential for B cell activation, particularly in response to antigens that have been tagged with complement proteins, allowing for an efficient immune response.
2. It serves as a co-receptor with CD19 and CD81, forming a complex that is critical for signaling pathways that lead to B cell proliferation and differentiation.
3. The binding of CD21 to complement C3d enhances the affinity of B cell receptors (BCRs) for antigens, increasing the likelihood of successful antigen recognition.
4. CD21 is also involved in the formation of germinal centers within lymphoid follicles, where B cells undergo affinity maturation and class switching.
5. Deficiencies or abnormalities in CD21 expression can lead to immunological disorders, such as increased susceptibility to infections and autoimmunity.

Review Questions

• How does CD21 function as a co-receptor in B cell activation?
  • CD21 functions as a co-receptor alongside CD19 and CD81 in B cells. When an antigen binds to the B cell receptor (BCR), CD21 enhances the signaling pathways triggered by this binding, leading to stronger activation and proliferation of B cells. This co-stimulation is crucial for effective antibody production in response to pathogens.
• Discuss the role of CD21 in enhancing the immune response through its interaction with complement components.
  • CD21 enhances the immune response by specifically binding to complement component C3d, which is attached to pathogens. This interaction strengthens the binding affinity between the antigen and the B cell receptor, facilitating more effective antigen recognition and promoting B cell activation. As a result, CD21 helps integrate signals from both the innate and adaptive immune systems.
• Evaluate how defects in CD21 could affect overall immunity and susceptibility to diseases.
  • Defects in CD21 can significantly impair the immune system's ability to respond effectively to infections. Individuals with compromised CD21 function may experience reduced B cell activation, leading to lower antibody production and higher susceptibility to infections. Additionally, such defects can contribute to autoimmune conditions due to a lack of proper regulation in B cell responses, ultimately resulting in a dysregulated immune system.

Related terms

B Cells: A type of white blood cell that plays a crucial role in the immune response by producing antibodies against antigens.

Complement System: A group of proteins that work together to enhance the ability of antibodies and phagocytic cells to clear pathogens from an organism.

Opsonization: The process by which pathogens are marked for destruction by the immune system through the binding of opsonins, such as antibodies or complement proteins.