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Immunology Ultimate Cheat Sheet

Over 1300+ words of extremely high-yield immunological concepts. Master Antibody structure, MHC Pathways, CD Markers, Toll-Like Receptors, and Hypersensitivity to guarantee maximum marks in the DBT BET examination.

1. Innate Immunity: PAMPs and PRRs

The immune system is broadly divided into Innate (non-specific, rapid, no memory) and Adaptive (highly specific, slower, lifelong memory) branches. The DBT-BET exam extensively tests your knowledge of how innate immune cells "recognize" pathogens through Pattern Recognition Receptors (PRRs).

Toll-Like Receptors (TLRs)

TLRs are a crucial class of PRRs found on macrophages and dendritic cells. They recognize highly conserved structures on microbes known as Pathogen-Associated Molecular Patterns (PAMPs). You must memorize which TLR binds to which specific microbial component, as this is a recurring matching question.

Receptor	Cellular Location	Specific Target Ligand (PAMP)	Microbial Origin
TLR-2	Cell Surface	Peptidoglycan / Lipoteichoic acid	Gram-positive bacteria
TLR-3	Endosomal Membrane	Double-stranded RNA (dsRNA)	Viruses
TLR-4	Cell Surface	Lipopolysaccharide (LPS)	Gram-negative bacteria
TLR-5	Cell Surface	Flagellin	Motile bacteria
TLR-9	Endosomal Membrane	Unmethylated CpG DNA	Bacteria and DNA Viruses

Memory Hack: TLRs that recognize nucleic acids (viral or bacterial DNA/RNA) are strictly located in the intracellular endosomes (TLR3, 7, 8, 9) to avoid recognizing self-DNA. TLRs that recognize structural proteins/lipids are on the cell surface.

2. Antibody Structure, Isotypes & Diversity

Immunoglobulins are heterodimeric glycoproteins synthesized strictly by plasma cells (differentiated B cells). The fundamental structure consists of two identical Heavy (H) chains and two identical Light (L) chains connected by disulfide bonds.

Figure 1: Standard Monomeric Antibody (IgG). Papain enzyme cleaves above the hinge yielding 2 Fab + 1 Fc. Pepsin cleaves below the hinge yielding 1 F(ab')2 + degraded Fc.

Immunoglobulin Classes (Isotypes)

• IgG: Most abundant in serum (80%). The only isotype that crosses the placenta to provide passive immunity to the fetus. Major antibody of the secondary immune response. Excellent at opsonization.
• IgA: The primary secretory antibody found in saliva, tears, breast milk (colostrum), and mucosal linings. Exists as a dimer connected by a J (joining) chain and protected by a Secretory Component.
• IgM: Exists as a massive pentamer (10 binding sites) connected by a J chain. It is the very first antibody produced in a primary immune response. It has the highest avidity and is the strongest activator of the Classical Complement Pathway.
• IgE: Present in minute quantities. Strongly binds to Fc receptors on mast cells and basophils. Mediates Type I Hypersensitivity (allergies) and defense against parasitic worms (helminths).
• IgD: Primarily functions as a B-cell receptor (BCR) on naive B cells along with IgM.

Generation of Antibody Diversity: V(D)J recombination creates millions of unique antibodies. RAG-1 and RAG-2 (Recombination Activating Genes) are the essential enzymes that cut and paste the Variable (V), Diversity (D), and Joining (J) segments. A mutation in RAG genes leads to Severe Combined Immunodeficiency (SCID), as neither B nor T cells can mature.

3. Major Histocompatibility Complex (MHC)

T-cells are virtually "blind" to free-floating antigens. They can only recognize protein fragments (peptides) that are formally presented to them on a specialized cellular tray—the MHC molecule. In humans, MHC is also called Human Leukocyte Antigen (HLA).

Feature	MHC Class I	MHC Class II
Expression	Present on all nucleated cells (absent on RBCs).	Present strictly on Antigen Presenting Cells (APCs): Macrophages, Dendritic cells, and B-cells.
T-Cell Interaction	Presents antigen to Cytotoxic T Cells (CD8+).	Presents antigen to Helper T Cells (CD4+).
Antigen Source	Endogenous (Cytosolic). E.g., Viral proteins synthesized inside the cell.	Exogenous (Extracellular). E.g., Bacteria engulfed via phagocytosis.
Structure	Alpha chain ($\alpha_1, \alpha_2, \alpha_3$) + $\beta_2$-microglobulin.	Two distinct chains: Alpha ($\alpha_1, \alpha_2$) and Beta ($\beta_1, \beta_2$).
Peptide Length	Accommodates short peptides (8-10 amino acids). Closed ends.	Accommodates longer peptides (13-18 amino acids). Open ends.
Key Chaperones	Proteasome, TAP transporter, Calnexin, Calreticulin.	Invariant chain (li), CLIP, HLA-DM.

Exam Trick: Use the "Rule of 8". MHC I $\times$ CD8 = 8. MHC II $\times$ CD4 = 8.

4. Hypersensitivity (Gell & Coombs Classification)

Hypersensitivity refers to an exaggerated or inappropriate immune response that causes tissue damage. Memorize the ACID mnemonic: Anaphylactic (I), Cytotoxic (II), Immune-complex (III), Delayed (IV).

• Type I (Immediate / IgE-mediated): Allergen cross-links IgE bound to mast cells, triggering massive histamine degranulation. Occurs in minutes.
  Examples: Anaphylaxis, Allergic Asthma, Hay fever, Peanut allergy.
• Type II (Cytotoxic / Antibody-mediated): IgG or IgM binds to antigens fixed directly on a cell surface, triggering complement lysis or ADCC (Antibody-Dependent Cellular Cytotoxicity).
  Examples: Erythroblastosis fetalis (Rh disease), Transfusion reactions, Myasthenia Gravis, Rheumatic fever.
• Type III (Immune Complex-mediated): Soluble Antigen-Antibody (IgG/IgM) complexes form in the blood and deposit in tissues (like kidneys or joints), activating complement and causing heavy neutrophil influx and inflammation.
  Examples: Systemic Lupus Erythematosus (SLE), Serum Sickness, Rheumatoid Arthritis, Post-streptococcal glomerulonephritis.
• Type IV (Delayed / Cell-mediated): The only hypersensitivity that is strictly antibody-independent. It is mediated by memory T-cells (Th1/macrophages) and takes 48-72 hours to manifest.
  Examples: Tuberculin (Mantoux) test, Contact dermatitis (Poison Ivy, Nickel allergy), Multiple Sclerosis, Type 1 Diabetes.

5. The T-Helper Dichotomy: Th1 vs Th2

When a naive CD4+ T cell is activated, it differentiates into either a Th1 or Th2 effector cell depending on the cytokine milieu. This balance dictates whether the body mounts a cell-mediated or humoral response.

• Th1 Cells: Driven by IL-12. They produce IFN-$\gamma$ and IL-2. They hyper-activate macrophages and promote Cytotoxic T-cell responses against intracellular pathogens (viruses, mycobacteria).
• Th2 Cells: Driven by IL-4. They produce IL-4, IL-5, and IL-13. They strongly stimulate B-cells to undergo class switching (especially to IgE) and activate eosinophils against extracellular parasites (helminths).

Crucial Concept: Th1 and Th2 mutually inhibit each other. IFN-$\gamma$ from Th1 suppresses Th2 expansion. IL-4 and IL-10 from Th2 completely suppress Th1 expansion.

Guaranteed Exam Hits

PYQ Direct Statements (High Yield Facts)

• Hybridoma Technology (Kohler & Milstein): Used to produce Monoclonal Antibodies. It involves fusing an immortal Myeloma cell (HGPRT deficient) with a mortal B-cell (spleen cell, HGPRT positive) using PEG.
  The HAT Medium Selection: Aminopterin in HAT blocks the De novo nucleotide synthesis pathway. Myeloma cells die because they lack HGPRT for the salvage pathway. Unfused B-cells die naturally. Only the hybridoma survives because it inherits immortality from the myeloma and the HGPRT enzyme from the B-cell.
• Complement Pathways Converge at C3: The Classical (Ag-Ab complex), Alternative (spontaneous C3 hydrolysis), and Lectin (Mannose-binding) pathways all converge at the formation of C3 Convertase. The ultimate goal is the formation of the MAC (Membrane Attack Complex: C5b-C9), which punches holes in bacterial membranes.
• Important CD Markers to Memorize:
  • CD3: Present on ALL mature T-cells (associated with TCR signaling).
  • CD19, CD20, CD21: Classic markers for B-cells. (CD21 is the receptor for the Epstein-Barr Virus).
  • CD56, CD16: Classic markers for Natural Killer (NK) cells.
  • CD14: Marker for Macrophages (acts as a co-receptor for TLR4 to bind LPS).
• Superantigens: Toxins (like Staphylococcal TSST-1) that cross-link the MHC Class II molecule directly to the TCR outside the normal peptide-binding groove. This bypasses specific antigen recognition, activating up to 20% of all T-cells simultaneously, leading to a massive, lethal "Cytokine Storm."
• Graft vs. Host Disease (GVHD): A unique transplant rejection scenario that occurs in bone marrow transplants. The transplanted immunocompetent T-cells from the donor recognize the recipient's tissue as foreign and attack the immunosuppressed host.
• Central Tolerance: Occurs in the Thymus. Positive Selection ensures T-cells can recognize self-MHC (must bind). Negative Selection ensures T-cells do not bind too strongly to self-peptides (prevents autoimmunity). The AIRE gene allows thymic cells to present tissue-specific antigens.

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