Immunology Key Concepts: Pathways, Organs, and Selection
1. Cytosolic vs. Endocytic Antigen Processing
| Characteristic | Cytosolic Pathway | Endocytic Pathway |
|---|---|---|
| Antigen origin | Intracellular (viruses, cytosolic proteins) | Extracellular (bacteria, ingested pathogens) |
| MHC involved | Class I | Class II |
| Activated cell type | CD8+ (cytotoxic T cells) | CD4+ (helper T cells) |
| Antigen processing | In the proteasome | In endosomes and lysosomes |
| Peptide transport | TAP (Transporter associated with processing) | From endosomes to MHC molecules in vesicles |
| Polymorphic zones | α1 and α2 | α1 and β1 |
2. Primary and Secondary Lymphoid Organs
| Organ | Type | Function |
|---|---|---|
| Bone marrow | Primary | Formation and maturation of B lymphocytes; production of hematopoietic stem cells. |
| Thymus | Primary | Maturation of T lymphocytes, positive and negative selection to prevent autoreactivity. |
| Lymph nodes | Secondary | Filter antigens; activation of B and T lymphocytes after antigen presentation. |
| Spleen | Secondary | Blood filtration; immune response to circulating antigens. |
| Mucosa-associated lymphoid tissue (MALT) | Secondary | Immune defense at mucosal barriers (respiratory, gastrointestinal, urogenital tracts). |
3. Thymocyte Selection: Positive, Negative, Neglect
- Double-positive thymocytes: Immature T cells expressing both CD4 and CD8 markers. This is an intermediate stage during their development in the thymus.
- Positive selection: Thymocytes that weakly recognize self-MHC molecules survive and differentiate into either CD4+ or CD8+ T cells.
- Negative selection: Thymocytes that strongly react to self-peptides are eliminated via apoptosis to prevent autoimmunity.
- “Die by neglect”: Thymocytes that fail to interact with any MHC molecules die due to lack of survival signals
4. True/False: Immunoglobulin Recombination
- “In heavy chain recombination of immunoglobulins, the V and J regions are directly joined”: FALSE. In heavy chains, the D region joins with the J region first, and then the V region joins the DJ segment.
- “Among the hypervariable regions in the variable domain of immunoglobulins, CDR3 is the most hypervariable”: TRUE. CDR3 is the most variable and critical region for antigen specificity.
5. MHC Class I and II: Structure and Polymorphism
| Characteristic | MHC Class I | MHC Class II |
|---|---|---|
| Chains | α (heavy) + β2-microglobulin | α and β, of similar sizes |
| Domains | α1, α2, α3 | α1, α2; β1, β2 |
| Peptide-binding groove | Formed by α1 and α2 | Formed by α1 and β1 |
| Polymorphic zones | Primarily in α1 and α2 | Primarily in α1 and β1 |
| Cellular expression | All nucleated cells | APCs (antigen-presenting cells) |
6. Immunology Definitions: Epitope, Antigen, Hapten
- Epitope: The specific part of an antigen recognized by an antibody or lymphocyte receptor.
- Antigen: A molecule capable of inducing an immune response by being recognized by antibodies or lymphocyte receptors.
- Hapten: A small molecule that is not immunogenic on its own but can elicit an immune response when attached to a carrier protein.
- Immunogen: A specific type of antigen that always elicits an immune response.