Adaptive Immunity and Antigens Quiz

Adaptive Immunity

• Adaptive immunity is an antigen-specific immune response.
• It involves two main types: humoral immunity and cell-mediated immunity.
• These types are mediated by different components of the immune system.
• They function to eliminate various types of microbes.

Antigens and Epitopes

• An immunogen is a protein or carbohydrate sufficient to initiate an immune response.
• An antigen is any molecule recognized by a specific antibody or T cell receptor (TCR), indicating its foreign nature.
• An epitope, or antigenic determinant, is the actual molecular structure that a single antibody or TCR interacts with.
• Epitopes can be either linear (a sequence of amino acids) or conformational (a three-dimensional structure).
• T cells primarily recognize linear epitopes.

Immunogens

• Immunogens are proteins or carbohydrates that stimulate an immune response.
• Immunogens may contain more than one antigen.
• Not all molecules are immunogens, yet all antigens are immunogens.
• Proteins are the most effective immunogens, while carbohydrates are weaker, and lipids and nucleic acids are poor immunogens.

Haptens

• Haptens are incomplete immunogens; they are too small to stimulate an immune response on their own.
• Haptens can become immunogenic when attached to a carrier molecule (e.g., a protein).

Adjuvants

• Adjuvants enhance the immune response to an antigen, prolonging antigen presence in tissue.
• Some adjuvants mimic microbial ligands for Toll-like receptors.
• Vaccines often use adjuvants to boost immune response.
• Adjuvants are not antigens.

Tolerance

• Tolerance is the unresponsiveness of immune mechanisms to a specific antigen.
• Central tolerance develops during fetal growth, generating unresponsiveness to self-antigens.
• Peripheral tolerance develops later in life for other proteins.
• For example, our immune system tolerates the food we eat.
• T-cell activation requires both antigen and co-stimulation.

Antigens (detailed)

• Foreignness: Molecules recognized as 'self' are not immunogenic. Immunogenicity requires 'non-self' recognition.
• Size: Larger proteins are usually the most potent immunogens. Small molecules (e.g. amino acids) are often weakly immunogenic or not immunogenic at all. Haptens only become immunogenic when bound to a carrier molecule.
• Complexity: Chemical complexity matters. Hetero-polymers are more immunogenic than simple homo-polymers.
• Genetic Constitution: Genetic makeup (e.g. different MHC alleles) influences the immune response to specific antigens.
• Dosage, route, and timing: Effective immune response often requires careful control of antigen dosage, administration route (oral, inhalation, parenteral), and timing.
• T-independent antigens: These antigens have a large, repetitive structure to directly activate B cells to produce antibodies (like IgM) without help from T cells. This response is usually limited and doesn't produce lasting memory.
• T-dependent antigens: Proteins are T-dependent antigens. They generate all five immunoglobulin types (classes) and yield a strong memory and booster response.

Major Histocompatibility Complex (MHC)

• MHC molecules were originally discovered as antigens responsible for tissue graft rejection.
• MHC molecules bind and present peptide antigens to T cells for recognition.
• In humans, MHC are called Human Leukocyte Antigens (HLA).
• Located on chromosome 6.
• MHC are essential for antigen presentation, crucial for T cell responses.
• There are MHC class I and class II, and class III molecules.

MHC Class I

• HLA-A, -B, and -C genes encode MHC class I proteins.
• These proteins are made of an alpha chain and a beta-2 microglobulin chain.
• Expressed on almost all nucleated cells.
• Important in presenting endogenous/cytosolic antigens to cytotoxic T cells.
• Includes genes for proteins involved in antigen processing.
• TAP transporters are parts of antigen presentation.

MHC Class II

• The HLA-D region encodes MHC class II proteins.
• Usually composed of alpha and beta chains.
• Primarily expressed on antigen-presenting cells (APCs) like macrophages, dendritic cells, and B cells.
• Essential to present exogenous antigens to helper T cells.

MHC Class III

• Encodes various components important to the immune system, like complement proteins and cytokines.

Antigen Processing and Presentation

• Antigen processing and presentation are necessary for T cell activation.
• Exogenous antigens (from outside cells) are internalized, processed, and presented by MHC class II.
• Endogenous antigens (from inside cells) are processed, and presented by MHC class I.
• Presentation of peptide-MHC complexes triggers T cell responses.
• Different mechanisms (e.g. proteasomes, TAP) lead to the production of antigenic peptides.

Cross-Presentation

• Some dendritic cells can process and present exogenous antigens via the MHC class I pathway.
• This allows them to activate cytotoxic T cells against viral or tumor-infected cells.

Presentation of Nonprotein Antigens

• Some T cell populations (e.g., NKT cells and γδ T cells) can recognize antigens like lipids, glycolipids, small phosphorylated molecules, or alkyl amines without needing MHC class I or II to present the antigen.
• These cells are "non-classical" in antigen presentation methods.