Immunology and Serology Review

Immunology

  • Study of host reactions to foreign substances.
  • Focuses on molecules, cells, tissues, organs, and systems that recognize and eliminate harmful substances.
  • Examines both desirable and undesirable immune system consequences.
  • Studies antigen and antibody behavior within the body.
  • Antigen: Foreign substance inducing an immune response.
  • Antibodies: Immunoglobulins produced by plasma cells in response to antigens.

Cells

  • B Cells:
    • Antibody producers.
    • Types: Plasma B cells, Memory B cells.
  • T Cells:
    • Coordinate immune response and kill infected cells.
    • Types: T helper cells, T cytotoxic cells, T regulatory cells.

Serology

  • Study of the noncellular blood portion (serum).
  • Diagnostic examination of blood serum related to the immune system's response to pathogens or introduced substances.
  • Study of antigen-antibody reactions outside the body, e.g., agglutination, precipitation.
  • Samples: Serum, CSF, Urine, Stool.

Serology Usage

  • When culturing infectious agents is not possible.
  • For confirmation of etiologic identification.
  • Diagnosis of immunologically related disorders.
  • Determination of immune status.
  • Detection of antigens or antibodies.
  • Diagnosis of cancers and infections.

Immunity

  • Process by which a host protects itself from external and internal threats.
  • Essential for defense against parasites, external elements, and sun exposure.

Key Terminologies

  • Antigen: Foreign substance that stimulates antibody production; large molecules (MW >10,000), usually protein or polysaccharide.
  • Antibodies (Immunoglobulins): Substances produced by immune system to block harmful antigens; produced by plasma cells.
  • Histamine: Vasoactive amine released during allergic reactions.
  • Hapten: Low molecular weight substance that can bind to antibody once formed.
  • Immunogen: Any substance capable of inducing immune response.

Antibody Functions

  1. Cell toxicity.
  2. Neutralization.
  3. Opsonization.

Antibody Types

  • IgG: Binds to phagocytes; main antibody for secondary responses; crosses placenta.
  • IgM: Fixes complement; main antibody of primary responses; B-cell receptor; immune system memory.
  • IgA: Secreted into mucous, saliva, tears, colostrum; tags pathogens for destruction.
  • IgE: Binds to mast cells and basophils; involved in allergy and antiparasitic activity.
  • IgD: B-cell receptor; stimulates release of IgM.

Immune System Roles

  • Defending the body against infections.
  • Recognizing and responding to foreign antigens.
  • Defending against tumor development.

Immunity Types

  • Innate Immunity:
    • Non-adaptive, non-specific, natural.
    • Ability to resist infections via normally present body functions; present at birth.
    • Response to foreign antigens remains the same each time.
  • Acquired Immunity:
    • Adaptive, specific. Host response depends on T and B lymphocytes; characterized by specificity, memory, and self vs. non-self recognition.
    • Acquired after birth; immune response developed after exposure to foreign antigens.

Immune System Mechanisms

  • Innate:
    • First Line: Skin, mucous membranes, secretions.
    • Second Line: Phagocytic leukocytes, antimicrobial proteins, inflammatory response, fever.
  • Acquired:
    • Third Line: Lymphocytes, antibodies, memory cells.

Types of Acquired Immunity

  • Active:
    • Natural: Infection (Antibody produced: Yes, Immediate Response: No, Duration: Long).
    • Artificial: Vaccination (Antibody produced: Yes, Immediate Response: No, Duration: Long).
  • Passive:
    • Natural: Transfer in vivo or colostrum (Antibody produced: No, Immediate Response: Yes, Duration: Short).
    • Artificial: Infusion of serum or plasma injection (Antibody produced: No, Immediate Response: Yes, Duration: Short).

Serology Section

  • Evaluates the body’s immune response by measuring antibody production and cellular activation.
  • Detects antibodies to bacteria, fungi, parasites, viruses, and autoantibodies.

Serological Tests

  • Agglutination: Involves particulate antigens.
  • Precipitation: Involves soluble antigens.

Serological Test Types

  • ELISA (Enzyme-Linked Immunosorbent Assay):
    • Measures enzymatic reactions.
    • Uses enzymes as labels.
  • Fluorescent Immunoassay: Uses fluorescent compounds as labels.
  • Radioimmunoassay: Uses radioactive substances as labels.
  • Flocculation Principle:
    • VDRL (Venereal Disease Research Laboratory): Screens for syphilis, uses heated serum.
    • RPR (Rapid Plasma Reagin): Screens for syphilis, uses unheated serum.

Flocculation vs. Agglutination

  • Flocculation:
    • Antigen Type: Soluble.
    • Reaction Medium: Liquid.
    • End Result: Fine precipitate (floccules).
    • Example: Syphilis tests.
  • Agglutination:
    • Antigen Type: Particulate antigen.
    • Reaction Medium: Liquid or solid surface.
    • End Result: Large clumps.
    • Example: Blood typing.

Common Viral Infections

  • Hepatitis A, B, C, D, E
  • HIV

Hepatitis Serological Markers

  • Hepatitis A: IgM anti-HAV, Total anti-HAV, HAV RNA
  • Hepatitis B: HBsAg, HBeAg, IgM anti-HBc, Total anti-HBc, anti-HBe, anti-HBs, HBV DNA
  • Hepatitis C: Anti-HCV, HCV RNA
  • Hepatitis D: IgM-anti-HDV, IgG-anti-HDV, HDV RNA
  • Hepatitis E: IgM anti-HEV, IgG anti-HEV, HEV RNA

HIV

  • Attacks the immune system, targeting T cells.
  • Two types:
    • HIV-1: Most common, virulent, and easily transmitted.
    • HIV-2: Less common, found in West Africa, less virulent, slower progression, more resistant to ART.

HIV Lab Testing

  • CD4 count
  • Detection of HIV Antibody: enzyme-linked immunosorbent assays
  • Screening test: ELISA
  • Confirmatory Tests:
    • Western Blot Testing or immunoblot, for HIV antibodies
  • Modern HIV algorithm: NAT and fourth generation ELISA
  • Viral load quantification: (1) reverse transcriptase assay, (2) indirect immunofluorescence assay, and (3) reverse transcriptase-polymerase chain reaction (RT-PCR) or branched-chain DNA (bDNA) amplification assays.

Syphilis Tests

  • Direct Microscopic:
    • Darkfield
    • Fluorescent antibody
  • Nontreponemal:
    • VDRL
    • RPR
    • TRUST
  • Treponemal:
    • FTA-ABS
    • MHA-TP or Serodia TP-PA

Dengue Testing

  • PCR DENV: Detect the viral genome in serum.
  • MAC ELISA: IgM antibody capture ELISA.
  • IgG ELISA: Detects a past dengue infection
  • NS1 ELISA: Detects NS1 antigen
  • PRNT: Most specific serological tool for determination of dengue antibodies

EBV Serological Test

  • Anti-VCA: IgM indicates current infection
  • Anti-EA IgG: Indicates current or recent infection
  • Anti-EA/D IgA: Indicates current or recent infection
  • Anti-EA/R: Appears in the acute phase
  • Anti-EBNA: Appears about 1 month after infection