Immunology - lecture 6

Adaptive vs. Innate Immunity

• Adaptive immunity utilizes specific receptors to recognize diverse molecules, while innate immunity relies on general recognition mechanisms.
• Adaptive immunity provides long-lasting protection against specific pathogens, while innate immunity provides immediate, but less specific, defense.

Cells of the Adaptive Immune System

• Lymphocytes are the primary cells of the adaptive immune system.
• B cells mature in bone marrow and are responsible for antibody production (humoral immunity).
• T cells mature in the thymus and contribute to cell-mediated immunity.
  • Helper T cells (CD4+) assist cytotoxic T cells and B cells.
  • Cytotoxic T cells (CD8+) directly kill virus-infected and damaged cells.

Antigen and Epitope

• An antigen is any molecule, or part of a molecule, recognized by the body as foreign.
• An epitope is the specific region of an antigen that binds to an antibody or T-cell receptor.

Antibody Structure

• Antibodies are composed of four polypeptide chains: two identical heavy (H) chains and two identical light (L) chains.
• The variable (V) regions at the N-terminus of each chain determine antigen specificity.
• The constant (C) regions at the C-terminus of each chain determine the antibody's biological function and isotype.
• The hinge region provides flexibility for binding to multiple antigens.
• Fragment antigen-binding (Fab) region binds to antigens.
• Fragment crystallizable (Fc) region interacts with other immune cells and molecules.

Antibody Isotypes

• Antibody isotypes are defined by differences in the constant region of the heavy chain.
• Five major isotypes include: IgM, IgG, IgD, IgA, and IgE.
• IgM is the first antibody produced during an immune response.
• IgG is the most abundant antibody in serum and provides long-lasting immunity.
• IgA is found in mucosal secretions and protects against pathogens entering the body.
• IgD functions as an antigen receptor on B cells.
• IgE is involved in allergic reactions.

Complementarity-determining Regions (CDRs)

• Hypervariable regions within the variable domains of antibodies form CDRs.
• CDRs make up the antigen-binding site and are responsible for antibody specificity.

Combinatorial Diversity

• Antibodies with differing specificities are generated through combinatorial diversity.
• This process involves assembling different V, D, and J gene segments to create unique variable regions.

T-Cell Receptor (TCR) Structure

• TCRs are transmembrane proteins that recognize antigen presented by MHC molecules.
• Most TCRs are composed of two chains: α and β.
• Each chain has a constant (C) region and a variable (V) region.

TCR Complex

• The TCR complex consists of the TCR, the CD3 complex, and co-receptors (CD4 or CD8).
• The CD3 complex is essential for TCR signaling and brings the TCR to the cell surface.
• CD4 co-receptor binds to MHC class II molecules.
• CD8 co-receptor binds to MHC class I molecules.

MHC Restriction

• T cells recognize antigen in the context of MHC molecules.
• MHC class I presents intracellular antigens to CD8+ T cells.
• MHC class II presents extracellular antigens to CD4+ T cells.

Summary

• Adaptive immunity is characterized by specific recognition of antigens through B-cell receptors (BCRs) and T-cell receptors (TCRs).
• BCRs are antibodies that recognize diverse antigens and trigger antibody production.
• TCRs recognize peptide antigens presented by MHC molecules, leading to activation of T cells for specific immune responses.
• The diversity of BCRs and TCRs is generated through complex mechanisms of gene recombination and somatic hypermutation.