Antibodies - Immunology - Medbullets Step 1

Overview

Introduction

Antibodies are immunologically active circulating proteins that
- are composed of two heavy chains paired with two light chains
- serve as a primary component of humoral immunity
- are produced by B-cells that can further
  - differentiate into plasma cells that specialize in secreting antibodies
  - mature to make antibodies with higher affinity
  - remain dormant as memory cells
- bind antigens from a wide variety of pathogens
- are also known as immunoglobulins (Ig)
Antibodies are able to fight infections through multiple mechanisms including
- opsonization of the surface of the pathogen leading to
  - phagocytosis by innate immune cells like macrophages
  - cytotoxicity by triggering release of toxic compounds by innate immune cells
- neutralization of pathogens and viruses by
  - blocking interaction of pathogenic proteins with host receptors
  - inactivating virulence factors expressed by pathogens
- activation of the complement cascade through the classical pathway

Antibody Structure

Antibodies are composed of two heavy chains paired with two light chains
Together these chains create distinct regions of the antibody such as
- the constant fragment (Fc)
- two identical variable antigen binding fragments (Fab)
These regions differ in both structure and function

Differences Between Antibody Regions
Feature	Antibody Binding (Fab)	Constant (Fc)
Composition	One heavy and one light chain Linked by disulfide bonds	Two heavy chains Linked by disulfide bonds Attached to carbohydrate chains
Terminus	N-terminus of protein chains	C-terminus of protein chains
Function	Binds to antigen in specific manner	Binds to complement proteins Binds to effector regions of innate immune cells
Variability	Site of idiotype diversity Changed in affinity maturation Unique for every antibody	Site of isotype diversity Changed in isotype switching 5 types that are shared among all antibodies

Antibody Variation and Diversity

Antibodies are able to fight an incredible range of infections because of
- idiotype diversity which
  - governs what antigens can be recognized by antibodies
  - is generated by multiple diversity mechanisms including
    - random recombination of VDJ regions of antibody coding regions
    - random addition of nucleotides to hypervariable regions by TdT
    - random assortment of heavy chains with light chains
    - affinity maturation through somatic hypermutation after antigen exposure
  - ensures that any moiety can be recognized by the variable region of an antibody
- isotype diversity through five types of constant regions


Antibody Isotypes
Feature	IgA	IgD	IgE	IgM	IgG
Constant chain type	α chain	δ chain	ε chain	μ chain	γ chain
Concentration	High in mucus membranes and in secretions	Low	Low	High in serum during early response	High in serum during late response
Valence	Dimer	Monomer	Monomer	Pentamer	Monomer
Function	Mucosal immunity Transported into mucosal lumens by poly Ig	Present on B-cell surfaces Unclear role	Defence against parasites Mediates allergies	Low affinity Main antibody in early response	High affinity Main antibody in late reponse
Recognized by	Innate immune cells Complement	Unknown	Mast cells Basophils	Innate immune cells Phagocytes Complement

Reference

Blood, Plasma, Serum	Reference Range
ALT	8-20 U/L
Amylase, serum	25-125 U/L
AST	8-20 U/L
Bilirubin, serum (adult) Total // Direct	0.1-1.0 mg/dL // 0.0-0.3 mg/dL
Calcium, serum (Ca²⁺)	8.4-10.2 mg/dL
Cholesterol, serum	Rec: < 200 mg/dL
Cortisol, serum	0800 h: 5-23 μg/dL //1600 h: 3-15 μg/dL 2000 h: ≤ 50% of 0800 h
Creatine kinase, serum	Male: 25-90 U/L Female: 10-70 U/L
Creatinine, serum	0.6-1.2 mg/dL
Electrolytes, serum
Sodium (Na⁺)	136-145 mEq/L
Chloride (Cl^-)	95-105 mEq/L
Potassium (K⁺)	3.5-5.0 mEq/L
Bicarbonate (HCO₃^-)	22-28 mEq/L
Magnesium (Mg²⁺)	1.5-2.0 mEq/L
Estriol, total, serum (in pregnancy)
24-28 wks // 32-36 wks	30-170 ng/mL // 60-280 ng/mL
28-32 wk // 36-40 wks	40-220 ng/mL // 80-350 ng/mL
Ferritin, serum	Male: 15-200 ng/mL Female: 12-150 ng/mL
Follicle-stimulating hormone, serum/plasma	Male: 4-25 mIU/mL Female: premenopause: 4-30 mIU/mL midcycle peak: 10-90 mIU/mL postmenopause: 40-250
pH	7.35-7.45
PCO₂	33-45 mmHg
PO₂	75-105 mmHg
Glucose, serum	Fasting: 70-110 mg/dL 2-h postprandial:<120 mg/dL
Growth hormone - arginine stimulation	Fasting: <5 ng/mL Provocative stimuli: > 7ng/mL
Immunoglobulins, serum
IgA	76-390 mg/dL
IgE	0-380 IU/mL
IgG	650-1500 mg/dL
IgM	40-345 mg/dL
Iron	50-170 μg/dL
Lactate dehydrogenase, serum	45-90 U/L
Luteinizing hormone, serum/plasma	Male: 6-23 mIU/mL Female: follicular phase: 5-30 mIU/mL midcycle: 75-150 mIU/mL postmenopause 30-200 mIU/mL
Osmolality, serum	275-295 mOsmol/kd H₂O
Parathyroid hormone, serume, N-terminal	230-630 pg/mL
Phosphatase (alkaline), serum (p-NPP at 30° C)	20-70 U/L
Phosphorus (inorganic), serum	3.0-4.5 mg/dL
Prolactin, serum (hPRL)	< 20 ng/mL
Proteins, serum

Hematologic	Reference Range
Bleeding time	2-7 minutes
Erythrocyte count	Male: 4.3-5.9 million/mm³ Female: 3.5-5.5 million mm³
Erythrocyte sedimentation rate (Westergren)	Male: 0-15 mm/h Female: 0-20 mm/h
Hematocrit	Male: 41%-53% Female: 36%-46%
Hemoglobin A1c	≤ 6 %
Hemoglobin, blood	Male: 13.5-17.5 g/dL Female: 12.0-16.0 g/dL
Hemoglobin, plasma	1-4 mg/dL
Leukocyte count and differential
Leukocyte count	4,500-11,000/mm³
Segmented neutrophils	54%-62%
Bands	3%-5%
Eosinophils	1%-3%
Basophils	0%-0.75%
Lymphocytes	25%-33%
Monocytes	3%-7%
Mean corpuscular hemoglobin	25.4-34.6 pg/cell
Mean corpuscular hemoglobin concentration	31%-36% Hb/cell
Mean corpuscular volume	80-100 μm³
Partial thromboplastin time (activated)	25-40 seconds
Platelet count	150,000-400,000/mm³
Prothrombin time	11-15 seconds
Reticulocyte count	0.5%-1.5% of red cells
Thrombin time	< 2 seconds deviation from control
Volume
Plasma	Male: 25-43 mL/kg Female: 28-45 mL/kg
Red cell	Male: 20-36 mL/kg Female: 19-31 mL/kg

Cerebrospinal Fluid	Reference Range
Cell count	0-5/mm³
Chloride	118-132 mEq/L
Gamma globulin	3%-12% total proteins
Glucose	40-70 mg/dL
Pressure	70-180 mm H₂O
Proteins, total	< 40 mg/dL

Sweat	Reference Range
Chloride	0-35 mmol/L
Urine
Calcium	100-300 mg/24 h
Chloride	Varies with intake
Creatinine clearance	Male: 97-137 mL/min Female: 88-128 mL/min
Estriol, total (in pregnancy)
30 wks	6-18 mg/24 h
35 wks	9-28 mg/24 h
40 wks	13-42 mg/24 h
17-Hydroxycorticosteroids	Male: 3.0-10.0 mg/24 h Female: 2.0-8.0 mg/24 h
17-Ketosteroids, total	Male: 8-20 mg/24 h Female: 6-15 mg/24 h
Osmolality	50-1400 mOsmol/kg H₂O
Oxalate	8-40 μg/mL
Potassium	Varies with diet
Proteins, total	< 150 mg/24 h
Sodium	Varies with diet
Uric acid	Varies with diet
Body Mass Index (BMI)	Adult: 19-25 kg/m²