12/01/25 B cells

B Cell Activation and Antibody Response

Introduction to B Cells and Antigens

B Cell Interaction with Antigens

  • B Cells recognize antigens through B cell receptors, specifically neuroglobulins.

  • When a B cell encounters an antigen (referred to as a microbe in this context), it engulfs (phagocytizes) the antigen and processes it.

  • The processed antigen is then presented through an MHC class II receptor to a T helper cell (CD4+ T cell).

Interaction Between B Cells and T Helper Cells

  • Recognition by T Helper Cells

    • The T helper cell binds to the antigen presented by the B cell, leading to a specific interaction characterized by the receptor-antigen interaction.

    • This interaction is strengthened by a cluster differentiation molecule on the T cell known as CD4.

  • Activation Process

    • This interaction activates the T helper cell, prompting it to release factors that subsequently activate the B cell.

Clonal Expansion of B Cells

  • Process of Clonal Expansion

    • Upon activation, B cells undergo significant nucleotide expansion—an increase in size due to substantial DNA replication.

    • This leads to an enlargement and proliferation of the specific B cell clones:

    • Clonal Expansion: All resulting B cells will produce antibodies that specifically target the same antigen that activated the initial B cell.

    • Types of B Cells Produced:

    • Memory B Cells: These cells retain receptors for recognizing the initiating antigen for future encounters.

    • Plasma Cells: Specialized cells equipped for high-rate antibody production.

Antibody Structure and Function

  • Basic Structure of Antibodies (Immunoglobulins)

    • Antibodies consist of four polypeptide chains: two heavy chains and two light chains.

    • The upper part of the antibody features antigen binding sites, crucial for the specificity of binding to antigens.

    • The crystallizable fragment (Fc) communicates the signal of the bound antigen to the immune system.

Antibody Variability and Specificity

  • Genetic Basis

    • Unique combinations of genetic fragments create diverse antibody structures, allowing for recognition of various antigens.

    • For** heavy chains**:

    • Composed of four different fragments, genes for which are located on chromosome 22.

    • Specificity arises from combinations of these fragments (e.g., B1, A1, J1, C1).

    • For light chains:

    • Consist of three different genes with multiple versions, which also affect binding affinity to antigens.

Functional Areas

  • The FAB region (antigen-binding fragments) is crucial for antigen recognition and binding.

    • The Fc region can bind different immune cells (e.g., eosinophils, mast cells) and induce distinct immune responses, including allergic responses or opsonization (enhanced pathogen recognition by phagocytes).

Mechanisms of Antibody Action

  • Activation of Complement

    • Antibodies can activate the complement system, leading to the formation of pores in pathogens, facilitating their destruction.

  • Opsonization

    • Antibodies adhere to pathogens, marking them for engulfment by macrophages through processes of opsonization.

  • Blocking Viral Receptors

    • Antibodies can prevent viruses from binding to host receptors, inhibiting infection.

  • Agglutination and Precipitation

    • Agglutination: Antibodies bind multiple cells, fostering their recognition as foreign by the immune system.

    • Precipitation: Antibodies bind to smaller molecules, facilitating their elimination from the body.

  • Neutralization of Toxins

    • Antibodies can bind and neutralize toxins, preventing interaction with host tissues (e.g., antidotes for snake bites produced via serum from animals exposed to sublethal doses).

Types of Immunoglobulins (Antibodies)

  • Immunoglobulin M (IgM)

    • A pentamer (five monomers), it is the first antibody produced in response to infections and is effective in activating complement.

  • Immunoglobulin E (IgE)

    • A monomer crucial for allergic reactions; it binds to mast cells and eosinophils, mediating responses against parasitic infections.

Kinetics of B Cell Response

  • Latent Period

    • Initial exposure to an antigen results in a latent period where no antibodies are detectable (approx. two weeks).

  • Primary Response

    • Following the latent period, IgM peaks after 7-10 days, followed closely by IgG production.

  • Secondary Response (Anamnestic Response)

    • Upon subsequent exposure to the same antigen, a more rapid and robust response occurs, reflecting the memory established by memory B cells, resulting in a higher antibody titer recorded in a log scale (e.g., $10^2$, $10^3$, $10^4$).

Conclusion and Further Discussion

  • Discussion on B cell activity, antibody production, and specific interactions continues, emphasizing the complexity and efficiency of the immune response in recognizing and combating pathogens.

  • The dynamic interplay between various cell types and their secreted factors is key in orchestrating an effective immune function against specific antigens.