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Humoral Immune Responses01:36

Humoral Immune Responses

Overview
Antigens Involved in Adaptive Immunity01:26

Antigens Involved in Adaptive Immunity

An antigen is any substance the immune system identifies as foreign and potentially harmful to the body, prompting an immune response. Antigens have two functional properties: immunogenicity and reactivity. Immunogenicity is the ability of an antigen to stimulate a specific immune response. At the same time, reactivity describes the antigen's ability to react with the cells and antibodies produced in response to it.
Complete Antigens
Complete antigens possess both immunogenicity and reactivity.
Antigen Presenting Cells01:22

Antigen Presenting Cells

The immune system is a complex network of cells and molecules that protects the body from foreign invaders. T cells, a type of white blood cell, play a crucial role in this process. They recognize and attack foreign substances, such as pathogens, that enter the body.
T cells require the help of antigen-presenting cells (APCs), which process foreign antigens into smaller fragments that can be recognized by T cells. These APCs are highly specialized cells that efficiently internalize antigens...
Antigen Processing Pathways01:31

Antigen Processing Pathways

MHC molecules are key players in the immune response, enabling T cells to recognize and respond to specific antigens. They are present on the surface of all nucleated cells in the body and are instrumental in presenting antigens to T cells and activating them. T cells recognize the MHC-antigen complex and initiate an immune response. MHC class I and MHC class II are two main types of MHC molecules, each associated with a distinct antigen processing pathway.
MHC Class I: Presenting Endogenous...
B Cell Activation and Differentiation01:24

B Cell Activation and Differentiation

The adaptive immune response, a sophisticated defense mechanism, relies on the activation and differentiation of B lymphocytes, or B cells. These processes enable our bodies to mount a tailored response against specific pathogens such as bacteria, free virus particles, toxins, and parasites.
When naive B cells encounter a specific antigen that can bind to the B cell receptor (BCR) on their surface, they undergo sensitization to respond to the antigen's presence. Sensitization begins with...
Antibody Actions01:26

Antibody Actions

Antibodies, or immunoglobulins, are critical players in the immune system's arsenal against invading pathogens. Produced by B cells and plasma cells, their primary role is to detect and bind to specific antigens, molecules found on the surface of pathogens like bacteria or viruses. Beyond antigen recognition, antibodies perform several vital functions that contribute to immune defense.
Neutralization
Antibodies can bind to pathogens, preventing them from infecting host cells. This process...

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Related Experiment Video

Updated: Jun 30, 2026

Detection and Enrichment of Rare Antigen-specific B Cells for Analysis of Phenotype and Function
09:25

Detection and Enrichment of Rare Antigen-specific B Cells for Analysis of Phenotype and Function

Published on: February 16, 2017

Natural antibody and complement-mediated antigen processing and presentation by B lymphocytes

B P Thornton1, V Vĕtvicka, G D Ross

  • 1Department of Microbiology and Immunology, University of Louisville, KY 40292.

Journal of Immunology (Baltimore, Md. : 1950)
|February 15, 1994
PubMed
Summary

Natural antibodies and complement C3 facilitate immune complex processing by B cells. This process involves complement C3dg, B cell receptors CR2 and FcRII, and enhances T cell activation.

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Area of Science:

  • Immunology
  • Biochemistry

Background:

  • Normal immune responses to novel protein antigens require complement C3 and its receptor CR2 (CD21).
  • Natural antibodies are crucial for initiating immune responses against previously unencountered antigens.

Purpose of the Study:

  • To investigate if natural antibodies against keyhole limpet hemocyanin (KLH) form immune complexes (ICs) that activate complement and promote B cell antigen processing via CR2.
  • To elucidate the roles of complement fragments (iC3b/C3dg) and B cell receptors (CR1, CR2, CR3, FcRII) in antigen processing and presentation.

Main Methods:

  • Detection of anti-KLH IgM and IgG in normal human sera.
  • Formation and characterization of immune complexes (ICs) with complement components.
  • Assessment of KLH processing and presentation by B cell clones using flow cytometry and blocking assays.
  • Analysis of co-stimulatory molecule (CD80) expression on B cells.

Main Results:

  • Natural IgM and IgG anti-KLH antibodies were detected, forming ICs that bound to B cells via complement (iC3b/C3dg) and receptors CR1, CR2, and CR3.
  • Complement fragment C3dg mediated KLH IC binding to CR2 on B cells, facilitating antigen processing.
  • B cell antigen processing and presentation to T cells were enhanced by KLH-specific B cells and involved CR2 and FcRII.
  • Binding of KLH ICs induced B7/BB1 (CD80) expression on B cells, dependent on IgG binding to FcRII (CD32).

Conclusions:

  • Responses to primary protein antigens involve natural IgG antibodies and complement C3.
  • B cell antigen processing and presentation are mediated by CR2 and FcRII interactions with immune complexes.
  • This pathway is critical for initiating adaptive immune responses to novel protein antigens.