B cells

 T cells
· Cell mediated immunity
· Originate in bone marrow & mature in thymus
· Bone marrow be T cell à thymus à Mature T cell à Th + Tc à direct interact with
    microbes
· Maturation involves = TCR, MHC, CD

Clonal Selection:
·  PLEASE : Preexisting Lymphocyte Exist Activated & expanded by Specific Epitope
 

IMMUNOGLOBULIN
· Gammaglobulin = Ig
· Basic formular: H₂L₂ , H= heavy chain; L = light chain
· Five major classes of antibodies = Depends on heavy chains
    · Mu (M), Gamma (G), Delta (D), Alpha (A), Epsilon (E)
    · Two types of light chain = Kappa, Lambda

· One Ig or Ab must contain same kinds of heavy & light chains.
· Each pair of H&L chains can bind 1 epitope. Since Ig must have two at least of such pair of
    H&L chain at a minimum it is bivalent
        · Except for IgM or tissue IgA
        · IgM valency = 10
        · IgA valency = 2-4

· Ig can exist as a receptor on B cell, or can be secreted as soluble protein
· J protein- present on IgM & IgA, serve to hold antibody structures together
· SCàsecretory componentà present on IgA- serve to prevent enzyme digestion
· Five kinds of heavy chains; there are 5 classes of Ab
    IgM- first Ab (1^o)
    IgD- not secreted, only as receptors on B cells
    IgG- major Ab (booster 2^o)
    IgA- mucosal Ab (major)
    IgE- allergy - related (trace)

· IgM: pentamer (in serum) (H₂L₂₎₅
· IgM: monomer (receptor- H₂L₂)
 

Functions of Ab
· neutralization: toxin inactivation
· precipitation: Ag- complex
· agglutination: cells expressing Ag-form clumps
· opsonization: Ab opsonies
· Bind complement (C1)

Different classes of Ab
IgM - 1 mg/ml, serum, half - life 5.5 days, does not cross plasenta; somewhat antiviral; not
    antitoxin; anti bacterial; highly agglutinating; non-allergic activate C'

IgG - 12mg/ml; half life 23 days; crosses plncenta & protects; very antiviral; antitoxin;
    antibacterial; to some extent agglutinating; non allergic; opsonizing; activate complement.

IgD - On B cells; 2-8 days
IgE - Reaginic antibody; 2 days 1/2 life; lowest concentration in blood; binds to mast cells &
    basophile and causes histamine release (in parasitic infection).
IgA - very abundant in mucosal infections.

Receptors on B&T cells

B cells:
Ig receptor (could be IgM or IgA monomer, IgG, IgD, Ig E), MHC

T cells:
· T cells do not interact with Ag directly, B cells do
· TCR = a , b chains
· N-terminal= variable; binds epitope (pieces of Ag) when brought by MHC protein (major
    histocompatiblity complex)

Two Types of T cells
 1. CD4: Recognizes epitope carried by MHC class II
 2. CD8: Recognizes epitope carried by MHC class I
 

Major histocompatibility complex: (MHC)
· MHC - represents closely linked genes present in all vertebrates species. Originally, proteins
    coded by these genes were found to distinguish self from nonself and cause rejection of
    transplants
· MHC proteins - responsible for transplantation rejection
    Xenograft – different species
    Allograft – same species
    Isograft – twins
    Autograft – same person

Now, it’s understood to play a vital role in presentation of Ag to T cell. T cell recognizes self or antigens only when presented with MHC molecules.
 

Two kinds of MHC:
MHC class I:
· present on all vertebrate cells
· MHC Class I: presents endogenously produced foreign antigens such as viral proteins.
· MHC class I consist of two of protein chains: alpha & b 2-microglobulin)

 · Class II: Involved in presenting foreign exogenous Ags.
· MHC class II present only on cells called Antigen Presenting Cells (APC) ® Macrophages,
    dendritic cells and B cells.
· MHC class II consist of two of protein chains: alpha & beta.