Antibodies (Human)

The ‘foot’ (bottom) of the antibody is known as the Fc fragment - binds to cells, binds to complement = effector function (kills or removes antigen)

The top (antigen binding) is the Fab fragment

Chains are held together with disulphide binds

Associated molecules allow intracellular signalling

Normally 3X constant heavy chain domains per chain and a hinge region (except μ and ε which have 4 and no hinge region)

Classes of Immunoglobulins

The five primary classes of immunoglobulins are IgG, IgM, IgA, IgD and IgE, distinguished by the type of heavy chain found in the molecule.

IgG - gamma-chains

IgMs - mu-chains

IgAs - alpha-chains

IgEs - epsilon-chains

IgDs - delta-chains.

Differences in heavy chain polypeptides allow different types of immune responses. The differences are found primarily in the Fc fragment. There are only two main types of light chains: kappa (κ) and lambda (λ), and any antibody can have any combination of these 2 (variation).

IgG

monomer

Gamma chains

70-85% of Ig in human serum.

secondary immune response

only class that can cross the placenta - protection of the newborn during first 6 months of life

principle antibody used in immunological research and clinical diagnostics

21 day half life

Hinge region (allows it to make Y and T shapes - increasing chance of being able to bind to more than one site)

Fc strongly binds to Fcγ receptor on phagocyte - opsono-phagocytosis

Activates complement pathway

IgM

Serum = pentamer

Primary immune responses - first Ig to be synthesised

complement fixing

10% of serum Ig

also expressed on the plasma membrane of B lymphocytes as a monomer - B cell antigen receptor

H chains each contain an additional hydrophobic domain for anchoring in the membrane

Monomers are bound together by disulfide bonds and a joining (J) chain.

Each of the five monomers = two light chains (either kappa or lambda) and two mu heavy chains.

heavy chain = one variable and four constant regions (no hinge region)

can cause cell agglutination as a result of recognition of epitopes on invading microorganisms. This antibody-antigen immune complex is then destroyed by complement fixation or receptor mediated endocytosis by macrophages.

In humans there are four subclasses of IgG: IgG1, IgG2, IgG3 and IgG4. IgG1 and IgG3 activate complement.

IgD

B cell receptor

<1% of blood serum Ig

has tail pieces that anchor it across B cell membrane

forms an antigen specific receptor on mature B cells - consequently has no known effector function (don’t kill antigens, purely a receptor) (IgM as a monomer can also do this)

IgE

Extra rigid central domain

has the most carbohydrates

IgE primarily defends against parasitic invasion and is responsible for allergic reactions.

basophils and tissue mast cells express very high affinity Fc receptors for IgE - mast cells then release histamine

so high that almost all IgE is bound

sensitizes (activates) mucosal cells and tissues

protects against helminth parasites

IgE’s main purpose is to protect against parasites but due to improved sanitation these are no longer a prevalent issue across most of the world. Consequently it is thought that they become over activated and over sensitive while looking for parasites and start reacting to eg pollen and causing allergies.

IgA

Exists in serum in both monomeric (IgA1) and dimeric (IgA2) forms (dimeric when 2 Fcs bind via secretory complex)

15% of the total serum Ig.

4-7 day half life

Secretory IgA2 (dimer) = primary defense against some local infections

Secreted as a dimer in mucous (e.g., saliva, tears)

prevents passage of foreign substances into the circulatory system

Isotype: class of antibody (IgD, IgM etc)

Allotype: person specific alleles

Idiotype: (hyper) variable region - antibody specificity

Antimicrobial Agents - Inhibition of DNA and Protein Synthesis

Bacterial chromosome replication

DNA replication

Bacterial Topoisomerases

maintain DNA in appropriate state of supercoiling

cut and reseal DNA

DNA gyrase (topoisomerase II) introduces negative supercoils

Topoisomerase IV decatenates circular chromosomes

these are the targets of the quinolone antibacterial agents

Quinolones

bind to bacterial DNA gyrase and topoisomerase IV after DNA strand breakage

prevent resealing of DNA

disrupt DNA replication and repair

bactericidal (kill bacteria)

Fluoroquinolone is particularly useful against

Gram +ves: Staphylococcus aureus, streptococci

Gram -ves: Enterobacteriacea; Pseudomonas aeruginosa

Anaerobes: e.g. Bacteroides fragilis

many applications e.g. UTIs, prostatitis, gastroenteritis, STIs

Adverse effects

Relatively well tolerated

GI upset in ~ 5% of patients

allergic reactions (rash, photosensitivity) in 1 - 2% of patients

Inhibition of Bacterial Protein Synthesis

Macrolides

in 1952: Erythromycin was isolated as the first macrolide (Streptomyces erythreus)

Newer macrolides: clarithromycin, azithromycin

Structurally they consist of a lactone ring (14- to 16-membered) + two attached deoxy sugars

Mode of action

bind reversibly to bacterial 50S ribosomal subunit

causes growing peptide chain to dissociate from ribosome → inhibiting protein synthesis

bacteriostatic (stops reproduction)

Macrolides’ spectrum of activity

good antistaphylococcal and antistreptococcal activity

treatment of respiratory & soft tissue infections and sensitive intracellular pathogens • e.g. Chlamydia, Legionella

Adverse effects

Generally well tolerated

nausea

vomiting

diarrhoea

rash

Aminoglycosides

large family of antibiotics produced by various species of Streptomyces (“mycin”) and Micromonospora (“micin”)

include: streptomycin, neomycin, kanamycin, gentamicins, tobramycin

Structure = linked ring system composed of aminosugars and an aminosubstituted cyclic polyalcohol

Mode of action of aminoglycosides

Bind irreversibly to 30S ribosomal subunit

disrupt elongation of nascent peptide chain

translational inaccuracy → defective proteins

bactericidal

Spectrum of activity

broad spectrum; mainly aerobic G-ve bacilli (e.g. P. aeruginosa)

used to treat serious nosocomial infections (hospital acquired infections)

First TB antibiotic

Used for cystic fibrosis

Adverse effects

all aminoglycosides have low Therapeutic Index (only a small amount needed to become toxic)

renal damage, ototoxicity, loss of balance, nausea

799 notes View Post

t-b-a-blr-blog

6 years ago

Antimicrobial Agents - Cell wall inhibitors

Based on mode of action • divided into families based on chemical structure

Modes of action Interference with:

cell wall synthesis

protein synthesis

nucleic acid synthesis

plasma membrane integrity

metabolic pathway

Inhibitors of Bacterial Cell Wall (peptidoglycan) Synthesis

The Beta-lactam Family

The Glycopeptides

Peptidoglycan is composed of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) repeat units, and amino acids. Each NAM is linked to peptide chain and the peptide chains are cross-linked.

β-lactams

Includes penicillin derivatives (penams), cephalosporins (cephems), monobactams, and carbapenems.

class of broad-spectrum antibiotics containing a β-lactam ring

Bacterial transpeptidase enzymes are responsible for catalysing cross-linking of the peptide chains

β-lactam ring bind to these transpeptidases – this inhibits cross-linking between peptide chains and prevents synthesis of stable PG

Cell wall synthesis ceases and the bacterial cells eventually die due to osmotic instability or autolysis.

Glycopeptides

Polypeptide agents - basic structural elements amino acids

Vancomycin:

complexes with peptide portion of peptidoglycan’s precursor units

vancomycin is a large hydrophilic molecule able to form hydrogen bonds with the terminal D-alanyl-D-alanine moieties of the NAM/NAG-peptides

preventing PG transglycosylation reaction – PG precursor subunits (NAG-NAM+peptide) cannot be inserted into peptidoglycan matrix;

Vancomycin also alters bacterial-cell-membrane permeability and RNA synthesis

Uses: serious Gram positive infections e.g. MRSA wound infection

Adverse effects:

damage to auditory nerve

hearing loss (ototoxicity)

“Red man/neck” syndrome - rash on face, neck, upper torso

590 notes View Post

t-b-a-blr-blog

6 years ago

126 notes View Post

t-b-a-blr-blog

6 years ago

Which Microbe Would You Be?

Buzzfeed has a quiz on what bacteria would you be, based on your personality.

I was a Salinicola salarius. (Which isn’t on my Periodic Table of Microbes!)

https://www.buzzfeed.com/grandon/which-bacteria-are-you-based-on-your-personality-3dk99?utm_term=.am4B9qO8B#.ab5oX2B0o

Follow on Twitter @warholScience

82 notes View Post

t-b-a-blr-blog

6 years ago

Positives are violet in color and negatives are red or pink on gram stain! My untidy handwritten notes here.

298 notes View Post

t-b-a-blr-blog

6 years ago

Hey Guys, Since My Previous Post On Morning Routines Was Very Well Received, I Decided To Make A Night

Hey guys, since my previous post on Morning Routines was very well received, I decided to make a night version. This small infographic thingy outlines the things I do at night to prep myself for rest, as well as some other things you could try to ensure you get a good night’s sleep. Hope this helps, and don’t hesitate to drop me an ask if you have any questions!

P.S. the typefaces used are Bromello and Montserrat

22K notes View Post

t-b-a-blr-blog

6 years ago