Seborrheic dermatitis is a scaly dermatitis on the scalp (dandruff) and face.
 - due to Pitysporium species
 - can be seen in AIDS as an opportunistic infection

Immunohistochemistry

 This is a method is used to detect a specific antigen in the tissue in order to identify the type of disease.

Group A Streptococcus
 - scarlet fever usually begins as a Streptococcal pharyngitis/tonsillitis and then develops an erythematous rash beginning on the trunk and limbs with eventual desquamation.
 - rash is due to elaboration of erythrogenic toxin by the organism
 - face is usually spared, but, if involved there is a characteristic circumoral pallor and the tongue becomes bright red, thus the term "strawberry tongue".
 - post-streptococcal immune complex glomerulonephritis is a possible sequela of scarlet fever.
 - Dick test is a skin test that evaluates immunity against scarlet fever; no response indicates immunity (anti-toxin antibodies present); erythema indicates no immunity.
 - impetigo due to Streptococcus pyogenes is characterized by honey colored, crusted lesions, while those with a predominantly bullous pattern are primarily due to Staphylococcus aureus.
 - cellulitis with lymphangitis ("red streaks") is characteristic of Streptococcus pyogenes.
 - hyaluronidase is a spreading factor that favors the spread of infection throughout the subcutaneous tissue unlike Staphylococcus aureus which generates coagulase to keep the pus confined.
 - erysipelas refers to a raised, erythematous ("brawny edema"), hot cellulitis, usually on the face that commonly produces septicemia, if left untreated. 

Chemical Mediators In Inflammation

Can be classified as :

A. Neurogenic

Also called the Triple Response of Lewis. It involves neurogenic vasodilatation of arterioles due to antidromic axon reflex arc. The constituents of the response are:

1. arteriolar vasoconstriction followed by

2. arteriolar vasodilatation

3. swelling

B. Chemical

1. Amines: Histamine and 5 hydroxytryptamine. Released  from platelets and mast cells.

Actions: Immediate and short lived.

Dilatation of arterioles.

Increased capillary premeability.

Kinins: Bradykinin and kallidin These are present in inactive from and are  activated by kinin forming proteases

Actions:

Arteriolar dilatation.

Increased vascular permeability

Pain

Kinin forming proteases Plasmin and Kallikrein. Present as inactive precursors.

Cleavage products of complement C₃a und C₅a are called anaphylatoxins

Actions:

Histamine release from mast cells

Chemotaxis (also C₅₆₇ )

Enhance phagocytosis.

 Polymorph components

Cationic: proteins which cause

Increased permeability

Histamine release.

Chemotaxis of monocytes

Neutral proteases which:

Cleave C3 and C5 to active form

Convert Kininogen to Kinin

Increase permeability.

Acid proteases which liberate leucokinins

Slow reacting. substance of anaphylaxis: (SRS-A) is a lipid released from mast cell.

Action --Increases vascular permeability

Prostaglandins: E1 + E2 _.

Platelets are rich source

Action:

Platelets are a rich source.

Vasodilatation.

Increased permeability.

Pain.

VIII. Miscellaneous: like

Tissue lactic acid.

 Bacterial toxins.

Erythema multiforme is a hypersensitivity reaction to an infection (Mycoplasma), drugs or various autoimmune diseases.
 - probable immunologic disease
 - lesions vary from erythematous macules, papules, or vesicles.
 - papular lesions frequently look like a target with a pale central area.
 - extensive erythema multiforme in children is called Stevens-Johnson syndrome, where there is extensive skin and mucous membrane involvement with fever and respiratory symptoms.

METAPLASIA

A reversible replacement of one type of adult tissue by another type of tissue. It is usually an adaptive substitution to a. cell type more suited to an environment, often at the cost of specialised function.

(1) Epithelial metaplasia:

• Squamous metaplasia. This is the commoner type of metaplasia and is seen in:
  • Tracheobronchial lining in chronic smokers and in bronchiectasis.
  • In Vitamin A deficiency.
• Columnar metaplasia:
  • Intestinalisation of gastric mucosa in chronic gastritis.

(2) Connective tissue metaplasia:

• Osseous-Metaplasia in :
  • Scars.
  • Myositis ossificans
• Myeloid metaplasia in liver and spleen.