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MI2.{5,7-8} | Immunodiagnostics & Immunological Disorders — SDL Guide (Part 2)

Immunodeficiency States

Primary (congenital) immunodeficiency results from genetic defects in immune components.

Deficiency type	Syndrome	Clinical features
B cell/antibody	X-linked agammaglobulinaemia (XLA)	Recurrent pyogenic infections after 6 months; absent B cells and Ig; absent lymphoid tissue
T cell	DiGeorge syndrome (thymic aplasia)	Recurrent viral, fungal infections; absent thymic shadow; hypocalcaemia (parathyroid agenesis); associated with 22q11 deletion
Combined T + B	SCID (Severe Combined Immunodeficiency)	Life-threatening infections with all pathogens from birth; absent T and B cells; "bubble boy"
Phagocyte	Chronic granulomatous disease (CGD)	Recurrent infections with catalase-positive bacteria (Staph, Aspergillus); defective NADPH oxidase (oxidative burst absent)
Complement	C3 deficiency, MAC deficiencies	(see previous module)

Secondary (acquired) immunodeficiency — most common causes in India:
- HIV/AIDS: Progressive CD4+ T cell depletion → combined immunodeficiency
- Malnutrition: Most common cause of immune suppression worldwide — protein-energy malnutrition impairs CMI, reduces complement and phagocyte function; highly relevant in rural India
- Diabetes mellitus: Impaired neutrophil function → recurrent skin/UTI/fungal infections
- Immunosuppressive therapy: Corticosteroids, methotrexate, cyclosporin (post-transplant, autoimmune disease)
- Malignancy: Haematological malignancies (CLL, myeloma) cause hypogammaglobulinaemia; solid tumours impair CMI

Laboratory evaluation of immunodeficiency:
- CBC with differential: Lymphocyte count; neutrophil count
- Immunoglobulin quantification (IgG, IgA, IgM) by radial immunodiffusion or ELISA — reduced in humoral immunodeficiencies
- Flow cytometry: CD4/CD8 ratio and absolute counts (HIV monitoring)
- Nitroblue tetrazolium (NBT) test / dihydrorhodamine (DHR) flow assay: Tests oxidative burst — absent in CGD
- Lymphocyte transformation test: Assesses T-cell proliferative response to mitogens

Transplantation Immunology

Transplant rejection is the major immunological barrier to successful organ grafting, driven by immune recognition of major histocompatibility complex (MHC/HLA) differences between donor and recipient.

Allorecognition pathways:
- Direct pathway: Host T cells directly recognise intact donor MHC molecules on donor antigen-presenting cells — responsible for acute rejection
- Indirect pathway: Host T cells recognise processed donor peptides presented on host MHC — responsible for chronic rejection

Types of rejection:

Type	Timing	Mechanism	Pathology
Hyperacute	Minutes to hours	Pre-formed donor-specific antibodies (ABO mismatch, prior sensitisation) → complement + thrombosis	Thrombotic vessel occlusion; graft immediately non-functional
Acute cellular	Days to weeks	CD8+ CTLs and Th1 cells attack graft	Lymphocyte infiltration, tubulitis (renal), endothelialitis
Acute humoral	Days to weeks	Donor-specific antibodies (DSA) → complement + Fc-mediated endothelial injury	C4d deposition, glomerulitis
Chronic	Months to years	Low-grade antibody + T cell responses → fibrosis	Graft vessel intimal thickening, interstitial fibrosis

HLA matching and crossmatch:
- HLA typing (by PCR or sequencing) identifies recipient and donor HLA-A, B, C (class I) and DR, DQ, DP (class II) alleles
- Crossmatch test: Donor lymphocytes + recipient serum → if pre-formed antibodies present → positive crossmatch → absolute contraindication to transplant (hyperacute rejection risk)
- Panel reactive antibody (PRA): Measures breadth of anti-HLA sensitisation in recipient; high PRA = harder to find compatible donor

Graft-versus-host disease (GvHD):
- Occurs in bone marrow/HSCT — donor T cells attack recipient tissues
- Acute GvHD: Skin, liver, gut involvement (cytokine storm, CTL-mediated)
- Chronic GvHD: Resembles autoimmune disease (scleroderma-like)

Immunosuppressive drugs in transplantation:
- Calcineurin inhibitors (cyclosporin, tacrolimus): Block IL-2 transcription, suppress T-cell activation
- mTOR inhibitors (sirolimus): Block T-cell proliferation
- Corticosteroids: Broad anti-inflammatory, suppress cytokine production
- Mycophenolate mofetil: Inhibits purine synthesis → suppresses lymphocyte proliferation
- Biologic agents: Basiliximab (anti-IL-2 receptor), belatacept (anti-CD28 co-stimulation blockade)

Tumour Immunology

Tumour immunology (cancer immunology) studies how the immune system recognises and destroys malignant cells, and how tumours evade this surveillance.

Tumour antigens:
- Tumour-specific antigens (TSAs): Unique to tumour cells; arise from oncogene mutations, viral proteins (EBV in Burkitt's lymphoma, HPV E6/E7 in cervical cancer)
- Tumour-associated antigens (TAAs): Present on tumour and some normal cells; often overexpressed (e.g., CEA in colon cancer, AFP in hepatocellular carcinoma, PSA in prostate cancer) — used as tumour markers

Immune surveillance (Burnet's theory): The immune system continuously monitors for and eliminates nascent malignant cells. Evidence:
- Immunocompromised patients (HIV, transplant recipients on immunosuppression) have dramatically higher rates of certain cancers (Kaposi's sarcoma, NHL, cervical cancer)
- Tumour-infiltrating lymphocytes (TILs) correlate with better prognosis in many cancers

Mechanisms of tumour immune evasion:
- MHC class I downregulation: Virally infected or mutated cells downregulate MHC I → CTLs cannot recognise them (NK cells can kill MHC I-deficient cells)
- PD-L1 upregulation: Tumours express programmed death ligand-1 (PD-L1), which binds PD-1 on T cells → T-cell exhaustion/anergy
- Immunosuppressive microenvironment: Tumour-secreted TGF-β, IL-10; recruitment of Tregs; IDO expression
- Antigen loss: Tumour cells that lose TSAs escape T-cell killing through immune selection pressure

Cancer immunotherapy (clinically relevant in India's tertiary centres):
- Immune checkpoint inhibitors: Anti-PD-1 (pembrolizumab, nivolumab), anti-CTLA-4 (ipilimumab) — block inhibitory signals, reactivate T cells against tumour
- CAR-T cell therapy: Patient T cells engineered to express chimeric antigen receptor targeting tumour surface molecule (e.g., CD19 in B-ALL)
- Cancer vaccines: HPV vaccines (Cervarix, Gardasil) prevent virus-induced cervical cancer; therapeutic vaccines (sipuleucel-T for prostate cancer)
- Monoclonal antibodies (mAbs): Trastuzumab (anti-HER2 in breast cancer), rituximab (anti-CD20 in B-cell lymphomas) — opsonisation + ADCC

Tumour Immune Evasion and Checkpoint Inhibitor Targets