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Primary Lymphoid Organs

Bone marrow- where B cells develop

Thymus (bilobed)- where T cells develop:
Progenitor cells from the bone marrow migrate to the thymus where the cortex contains immature thymocytes, which mature and migrate to the medulla
In the medulla they learn to discriminate between self and non-self during fetal development and short time after birth, then leave the medulla and enter the peripheral blood circulation where they are tranported to the secondary lymphoid organs
Reaches its maximum size and activity at birth
Atrophies with age –almost disappears at puberty

DiGeorge syndrome:
Congenital absence of thymus results in an immediate and drastic reduction in T cells that produces a potentially lethal wasting disease


Secondary Lymphoid Organs

The secondary lymphoid organs have two major functions:
1. Trap and concentrating foreign substances
2. Main sites of production of Abs and T cells

The major secondary lymphoid organs include:
Spleen- responsive to blood-borne antigens
Lymph nodes- protect the body from antigens that enter from the skin & internal surfaces via the lymphatic system

Mucosa-associated lymphoid tissue (MALT):
Scattered along mucosal linings
Protect against Ags entering the body directly through mucosal surfaces


Spleen: Anatomy

White pulp: has many WBCs and this includes T and B cells, and when the B cells mature they leave and go to the red pulp and are associated with RBCs and other cells
The spleen is important for phagocytosis facilitated by Ab and other opsonization methods, therefore you find phagocytosis of encapsulated cells is important to happen in the spleen because without the spleen it is difficult for phagocytes to capture these encapsulated cells
People without spleen, frequent infections with encapsulated bacteria


Lymph Nodes

Lymph nodes are clustered in strategic points in the body: neck, axillae, groin, mediastinum, abdominal cavity
Filter Ags from interstitial tissue fluid and lymph during its passage from the periphery to the thoracic duct

Two types:
Somatic nodes: Lymph nodes that protect the skin
Visceral nodes: Deep lymph nodes protecting the respiratory, digestive, and genitourinary tracts



More than 50% is lymphoid tissue
Mucosa-associated lymphoid tissue is composed of
GALT- line the intestinal tract (Gut)
BALT line the respiratory tract (Bronchus)
Genitourinary tract lining lymphoid tissue

Secretory IgA (sIgA)
Major effector mechanism; only Ab that can be secreted out of the body/tissue
Secreted directly onto the mucosal epithelial surfaces


Examples of MALT

Peyer patches that line the small intestine and mainly secrete IgA; always the cells that makes IgA regardless is the B cell

M cells that are associated with the intestine and the purpose of their existence is the increase the surface area that is exposed to the nutrients within the intestines; they have microfolds on only one side of the intestine to increase the absorption of the nutrients
Absorb, transport, process, present antigens to subepithelial lymphocytes


Polyclonal Ab

Polyclonal Abs are a heterogeneous mix of Abs that recognize multiple epitopes but Ab specificity against the same Ag (many Ab, one Ag); recognizes many epitopes on only one type of antigen
Produced by variety of Ab-producing cells
They are many clones of cells
Polyclonal Abs recognize & react against different epitopes on the Ag
Avidity- sum of the affinities to the different epitopes or the sum of the members of the polyclonal group
Isolate them by growing them in a mouse


Monoclonal Ab

Monoclonal Abs are produced by a single clone of cells (only one B cell)
Resultant Abs are identical in all aspects with the same affinity and same binding specificity
Recognize the same epitope
They are produced in hybridoma between activated B cells and malignant plasma cells (fusion) so lengthen the live of the B cells and give the myeloma cells Ab producing abilities



Antigen binding sites in the variable region
Constant part of light chain doesn’t really have a function associated with it, but the heavy chain part is the Fc fragment where we treat the molecule with papain that is an enzyme that can break the proteins on the certain site and get two identical fragments of Fab, which maintains the function of the Ab because they contain the Ag binding site needed to bind Ag
The Fc fragment (H/heavy chain) crystallizes in the cold and can bind to Fc receptors on other cells and so if you have a complete Ab by binding to the antigen specifically, then the Ab can bind to the cell (phagocyte, macrophage, NK, etc) and this will bring the antigen closer to the cell so it can be phagocytosed and then destroyed
Heavy Chain Designation: gamma
Subclasses: 4
Biologic Activity: opsonin, fixes complement, crosses placenta



IgM heavy chain has an extra domain compared with IgG
J chain- small cysteine-rich protein that initiates cross linking of C3 and C4 of 5 IgM monomers producing the pentameric form of IgM found in circulation

Cysteine rich regions to form disulfide bridges to combine the chains together and bind the whole complex together
When the B cells mature, they produce IgM first; if the B cells matures further in a process called Class Switching, it changes to other Ig’s; the change must keep the specificity the same or it doesn’t serve the purpose; the variable part does not change and the constant region will change
Heavy Chain Designation: mu
Subclasses: 1
Biologic Activity: fixes complement, agglutination, antigen receptor, indicative of acute infections



IgA2: secretory IgA is dimeric and held together by J chain and secretory component
IgA1: serum IgA is monomeric
Heavy chain has three constant regions
Only Ig that is secreted outside of the body/tissue to fight infections in the mucosa in the body; eye (conjunctivitis), intestine, respiratory system
Heavy Chain Designation: alpha
Subclasses: 2
Biologic Activity: secretory Ab and agglutination



IgE is similar to IgG
Has 4 constant region domains on the H-chain like IgM
Defense against parsitic infestation
Heavy Chain Designation: epsilon
Subclasses: 1
Biologic Activity: anti-parasitic and allergy Ab



Similar to IgG in structure, but NEVER secreted
Functions as a receptor on the B cell always
Variable parts have specificity and bind certain antigens; determines which Ig that is released from the B cell
Part of the receptor/signaling complex of B cells
Heavy Chain Designation: delta
Subclasses: 2
Biologic Activity: antigen receptor


Primary Ag-Ab Reaction

The first interaction between Ag & Ab
Key-lock principle like enzymes, but doesn't need an enzyme
Ag-Ab interaction is precise = specific
Characteristics of Ag-Ab reaction:
i. Rapid, in seconds
ii. Independent of electrolytes, salt, buffer
iii. Not visible


Chemical Interactions for Ab-Ag Reactions

Four noncovalent interactions hold antigenic determinants w/in Ab-binding site:
1. Coulombic (electrostatic, ionic) interactions
2. Van der Waals forces- weak attractive force between atoms or nonpolar molecules caused by a temporary change in dipole moment arising from a brief shift of orbital electrons to one side of one atom or molecule, creating a similar shift in adjacent atoms or molecules
3. Hydrogen bonds
4. Hydrophobic interactions


Affinity and Avidity

Affinity is the strength of Ag-Ab bonds between a single epitope and an individual Ab’s binding site
Avidity: The binding strength between a multivalent Ab (polyclonal Ab) and a multivalent Ag

K = [Ag..Ab]/[Ag][Ab] = Ab affinity/association constant
K = Associated complex/dissociated Ag & Ab
Dissociation constant: 1/K, meaning that you put the [Ag] [Ab]/ [Ag….Ab]; the smaller the amount of D, the higher the affinity; reverse of other equation
The higher K, the higher the association between Ab-Ag


Secondary Ag-Ab Reaction/Response

Secondary Ag-Ab reaction: the conversion of the invisible primary reactions macroscopically visible ones as in the case of precipitation and agglutination

Secondary response: the immune response which follows a second encounter with a particular Ag


Lattice Formation

Occurs when Ag-Ab complexes aggregate in form of precipitation in liquid medium - or -
Agglutination, including particulate components, other than Ag and Ab, such as cells

A regular arrangement of units into an array such that a plane passing through two units of a particular type or in a particular interrelationship will pass through an indefinite number of such units; e.g., the atom arrangement in a crystal.
Reference to a complex between several Ab and Ag at the same time instead of one Ab to one Ag


Cytotoxic T cells

Cytotoxic T cells kill infected cells to prevent pathogen production
Cytotoxic T cell receptors detect viral protein fragments on the surface of infected cell
Large numbers of virus specific cytotoxic T cells are produced
EBV infection in B cells cause Tc cells to make up the vast majority of WBCs


Resting vs. Active B cells

No Gall bodies
No LGL morphology
Monoribosomes scattered in the cytoplasm
Activated B cells have rough ER whereas inactive ribosomes aren't attached to the ER


T cell Independent Defense Mechanisms



Complement Cascade: Classical Pathway

Classical: requires IgG in a complex with Ag, or IgM; these are the activators; this pathway is associated with acquired or adaptive immunity
Activated after the Alternative pathway via Antigen-IgG (1-3) complex, pentameric IgM
Non-Activators: IgG4, IgA, IgE because they lack C1q receptors and cannot bind C1q
C3b is produced and that point on is the same for both pathways and goes on until it produces the membrane attack complex and destroys the cell


Complement Cascade: Alternative Pathway

Alternative: does not require Ab to be activated

activated by components of the infectious agents like cell wall of bacteria, yeast, fungi, LPS, aggregated IgA, cobra venom; NO need for Ab-Ag complex or IgM for activation

This pathway is the first pathway to be activated because only the presence of the bacterium itself is sufficient enough to activate it, whereas the classical needs Ab in order to be activated

C3b is produced and that point on is the same for both pathways and goes on until it produces the membrane attack complex and destroys the cell



Non-immunoglobulin serum proteins
Involved in:
i. Control of inflammation
ii. Stimulation of phagocytosis
iii. Activation of cell lysis
Cascade of different proteins that act upon each other as enzymes, and this system is designed to cause lysis of certain cells like bacteria, or own body cells sometimes as well
Defense: opsonization and stimulation of phagocytosis and inflammation process



Preparation for eating/ phagocytosis facilitating process
Opsonins are deposited on the Ag
Examples: IgG and complement
Bacteria can be labeled with IgG or complement fragments
C3b or intermediate C3b or C4b can bind to the bacterium and bind to CR receptor so the bacteria is captured and swallowed by the phagocyte


Complement Cascade: After Pathways Fuse

We must know that the component C3b is necessary for activation and comes from C3; C3b by itself is always present in the serum at low concentrations (alternative pathway)
C3b can bind foreign substance like LPS, and this complex of LPS-C3b can bind to the factor B, which is present in the serum and then we have a complex that is called C3bB (factor B is second B); factor B acts as enzyme which causes separation of b and B so end product: c3bBb (C3 convertase enzyme) to split C3bB to get more C3b
C3b has an anaphylactic role and vital to many cells
End product is C3 convertase that produces C3b which binds to the same convertase to become C5 convertase (C3bBb3b)
Only part of the C3b makes C5 convertase and the rest of the C3b is destroyed by the factors I and H, why? If we have a lot of C3b in the system and serves no function to make C5 convertase, then the C3b has to go somewhere; it is an opsonin and bind any cell in the body and tag them for phagocytosis and cause a lot of damage, so need to get rid of it
Start with small amount of C3b in serum and need to generate more to make it more sufficient aka amplification


Sites of Complement Synthesis

As opposed to Ig Abs which are synthesized in B (plasma) cells only, complement proteins are synthesized in hepatocytes, macrophages (various tissues), epithelial cells (GI tract), and monocytes in blood


Functions Complement

Direct killing of bacteria & virus-infected cells by lysis mediated by MAC, which inserts itself in the membrane of the target to make holes to cause lysis of the cell

Indirect killing by opsonization followed by phagocytosis & intracellular killing
Immunization does NOT increase complement concentration in the serum
part of the innate immune system


Steps to Activate the Classical Pathway

Step 1: Activation of C1- C1q + C1r +C1s (C1r & C1s are proteases, activate C4 & C2)
Step 2: Activation of C4- C4a +C4b; C4a is soluble and leaves, while C4b is stable
Step 3: Activation of C2- C2a + C2b; C2b is the soluble part that leaves, while C2a is stable
Step 4: Formation of C3 convertase- C4b2a
Step 5: Activation of C3- C3a + C3b
Activation of C5- C5a + C5b by C5 convertases:
The classical C4b2a3b or the alternative C3bBb3b

Classical and Alternative the same:
C5b + C6 + C7  C5b67 .. + C8 C5b678
Adding 6 molecules of C9 to C5b678= MAC C5b678(9)6
making holes in the membrane and the cell lyses


Split Products of the Complement Cascade with Activity

C4b and C3b = opsonins
C3a and C5a = anaplylatoxins (mast cell degranulation)
C5a = chemotaxis which attracts neutrophils