Humoral immunity (antibody structure and classification)

T Cells and cell-mediated immunity

• Inherited; serves as a first line of defense 
• External defense
• Skin
• Epithelial barriers 
• Digestive tract (HCl, enzymes) 
• Respiratory tract (mucus, cilia, enzymes) 
• Genitourinary tract (acidity) 
• Internal defense – phagocytes, fever, interferons

• Lipopolysaccharide (LPS) from Gram “–” bacteria cell membrane
• Peptidoglycan from Gram “+” bacteria cell wall

cytokines

mast cells

 (1) dilation of blood vessels → ↑ membrane permeability → recruit blood monocytes to damaged area → macrophages → phagocytosis

(2) constriction of bronchi & bronchioles

• First to arrive are neutrophils (pus formation) 

• Pseudopod movement→vacuole formation →fuse with lysosomes → phagolysosomes 
• Tissue damage → cell necrosis → expressed danger-associated molecular patterns (DAMPs) → ↑ innate immunity & inflammation 
• Cells die by apoptosis do not express DAMPs

• Body temperature is regulated by hypothalamus 
• Can be induced by endogenous (IL etc.) or exogenous pyrogens (LPS) 
• Endogenous pyrogens – IL1-β & other cytokines, released by WBCs and brain in response to endotoxin from gram “–” bac (LPS) 
• Nonspecifically | bac; ↑ neutrophil activities; ↑ interferon production

• Polypeptides produced by cells infected with virus
• Provide short-acting, non-specific resistance to viral infection in nearby cells

• ↑ Overall immune activities by ↑  phagocytosis, ↑  killer T cell activities, ↑  natural killer cell activities, ↑ Ab production by plasma cells
• ↓ tumor growth, ↓ cell division, ↓ maturation of adipocytes, RBC

• Humoral immunity
• Cell-mediated immunity

molecules that elicit production of Ab that specifically bind to Ag

• Usually large molecules that are foreign to the body

B cells

T cells

• Named after lymphocytes processed from chicken bursa of Fabricius
• Memory cells and plasma cells 

• Of thymus origin 65-85% of blood lymphocytes are T cells 
• Killer (cytotoxic) T Cells, helper T cells, suppressor T cells 
• HIV virus specifically attacks helper T cells.

• Ab on surface that are receptors for Ag
• Ag-Ab binding → B cell clone expansion → production of Ab and activation of complement proteins

Memory cells + Plasma cells

• Some offspring of B cell÷=memory cells (long-lived)
• Plasma cell factory churns out ~2000 antibodies/sec

• In shape of “Y” 
• 2 long heavy (H) chains are joined to 2 shorter light (L) chains 
• Each chain contains constant (or crystallizable) (Fc) and variable fragments (Fab) 
• Fab confers Ab specificity 
• Ab diversity – 1020 Ab/person

• IgG – main Ab in circulation 
• IgA – main Ab in external secretions in mucosal area 
• IgE – responsible for allergic symptoms 
• IgM and IgD – as Ag R’ on lymphocytes

• Recombination of genes code for Hs and Ls in developing lymphocytes → Ag-independent diversity 
• Diversity further increases via somatic hypermutation → B cells undergo Ag-dependent proliferation

• Carry CD8 cell surface marker 
• Kill pathogens by cell-mediated destruction – in contact with victim cell 
• Secrete perforins → create a pore in victim's membrane → lysis 
• Secrete granzymes → destroy victim's DNA

• Carry CD4 surface marker 
• ↑ Responses of both killer T cells and B cells 
• HIV infects helper T cells and macrophages

• Carry CD25 surface marker (and CD4) 
• ↓ Responses of killer Ts and B cells 
• Help protect against autoimmune responses

secreted by lymphocytes

• Usually called interleukin (IL)-1, 2, 3

• Require prior exposure to the specific antigen such as vaccinations 
• Eg. Jenner’s cowpox inoculation against smallpox 
• Vaccines may use (1) live attenuated virus, (2) killed virus, or (3) recombinant viral proteins

• Donor was given active immunity, Ab was transferred to recipients 
• May develop allergic response if Ab transfer in multiple times

• Second line of arrival – monocytes, become macrophages and may serve as antigen presenting cells 

• Third line – T lymphocytes, for specific immunity