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MedBio
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Wholly responsible for the specific immune recognition of pathogens (initiate adaptive immune responses
All lymphocytes are derived from bone marrow
T lymphocytes – thymus B lymphocytes – bone marrow
Spleen Lymph nodes Appendix Tonsils Other Mucosal associated lymphoid tissues (MALT)
Recognizes a particular Ag using a receptor molecule on its surface
Different types and have a variety of functions: 1. Regulating immunoglobulin production (TH)
2. Interacting with mononuclear phagocytes and
helping them destroy pathogens (TH)3. Lysing virally infected cells ( T Cytotoxic cells-Tc)
4. Delayed hypersensitivity
Mononuclear phagocytes with cytotoxic ability
Have the capacity to recognize the surface changes in tumor cells and virally infected cells
Use recognition systems which are non-specific
NK cells
FUNCTION T cell Phenotype (65%-35%)
T1+ T3+ T4+ T11+
T1+ T3+ T8+ T11+
Effector cells for delayed Hypersensitivity
Effector cells for cytotoxicity
Help for immunoglobulin Synthesis
Help for cytotoxicity
Suppressor for Ig synthesisand delayed hypersensitivity
Inducer of suppressor
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Stimulation of lymphocytes by antigen or mitogens (substances which caused lymphocytes to undergo cell division)
Followed by proliferation and differentiation of the progeny into various effector and memory cells.
3 Major Categories of Molecules that Trigger the Activation of Lymphocytes
1) Monoclonal activators – antigens (Ag)
2) Oligoclonal activators- superantigens
3) Polyclonal activators - mitogens
Antigens Stimulate/activate T cells or B cells
after contact with an antigen through their receptor specific for an epitope.
Result: generation of one clone of T cell and B cell population.
T-cell dependent Ag Antigens (Proteins) will only activate B
cells in the presence of T cells (and the proteins that they secrete).
T cell recognize the antigen cytokines are released B cells are stimulated
T- cell independent Ag B cell directly recognizes the antigen
(Polysaccharides ) Antigens that can activate B cells in the
absence of T cells
Superantigens Molecules typically derived from bacteria
Activate a subset of T cells but not all T cells
T cell recognizes particular amino acid sequences (epitopes) that are common to its receptor.
Result: activated cells generate a few, different clones
Mitogens Epitopes are recognized virtually by all
T cells and B cells Result: whole populations of T cells and
B cells are activated and many clones are generated
Plant proteins – polyclonal activators Phytohemagglutinin (PHA) and
Concanavalin A (Con A) are plant glycoproteins activate T cells
Lipopolysaccharides (LPS) activate B cells
Pokeweed mitogens (PWN) activate both T cells and B cells.
Polymorphonuclear leukocytes (PMN): basophils, neutrophils and eosinophils
Monocytes Macrophages Natural Killer cells Antigen presenting cells (APC) –
dendritic cells and macrophages Ag recognition is non-specific
Direct recognition – immediate encounter and recognition of a cell Ag is interacting directly with a receptor
present in the cell. Examples:
Phagocytes recognize a bacterium phagocytes have primitive pattern
recognition receptors that interact with
many microorganisms.
Indirect recognition another molecule (opsonin) binds
to the antigen Opsonin serves as a link between
the cell and the antigen. Examples:
Phagocytes that express receptors for opsonins
Eosinophils that express receptors for IgE antibodies that bind a helminth
Natural killer cells that express receptors for IgG antibody that bind a virally infected cell.
T cells and B cells have antigen-recognizing receptors that bind antigen but interact only with the epitopes.
CellsRecognize epitopes on
Antigen recognizing receptor
B cells Intact antigenMembrane-bound antibodies
T cellsAntigen fragments bound to MHC
T cell receptor (TCR)