(2) Adaptive Immune System Main Components

Adaptive immune system

the body’s specific, learned defense mechanism that develops after exposure to pathogens or vaccines

Adaptive immune system speed

slower to activate, days after first exposure; responds faster and stronger upon re-exposure

Lymphoctyes

B and T cells that originate from hematopoietic stem cells in bone marrow

B cell site of maturation

bone marrow

B cells

produce antibodies (humoral immunity)

T cell site of maturation

Thymus

T cells

coordinate or kill infected cells (cell-mediated immunity)

Where do lymphotcytes go after maturation?

circulate through blood and lymph to populate secondary lymphoid organs

Antigen

a foreign molecule that triggers an immune response

Epitope

small portion of antigen recognized by a receptor

Antigen-presenting cells (APCs)

macrophages, dendritic cells, and B cells present antigens on MHC molecules to T cells

Humoral immunity

B-cell mediated, targets extracellular pathogens (bacteria, toxins, viruses in blood or mucosa)

Humoral immunity steps

1. B cell receptors bind specific antigens, often require T helper cell

2. T helper cells release cytokines, stimulating B cell proliferation and differentiation:

   1. Plasma cells

   2. Memory B cells

Plasma cells

secrete antibodies (IgM, IgG, IgA, IgE, IgD)

Memory B cells

persist long-term for rapid response to reinfection

Antibody neutralization

bind and block toxins or viruses

Antibody opsonization

coat pathogens for phagocytosis

Antibody complement activation

trigger the classical complement cascade

Antibody agglutination

clump microbes for easier clearance

Primary humoral immunity response

slow, mainly IgM

Secondary humoral immunity response

faster, stronger, mainly IgG

Cell-mediated immunity

T-cell mediated, targets intracellular pathogens and abnormal host cells

T cell types

Helper T cells (Th), Cytotoxic T cells (Tc), and regulatory T cells (Treg)

Helper T cell surface marker

CD4+

Cytotoxic T cell marker

CD8+

Regulatory T cell marker

CD4+, CD25+

Helper T cell function

“managers”, activate B cells, macrophages, and cytotoxic T cells

What type of antigens do helper T cells recognize?

MHC II complex

Cytotoxic T cell function

kill infected or abnormal cells directly by releasing perforin and granzymes to trigger apoptosis of target

What type of antigen do cytotoxic T cells recognize?

MHC I complex

Where is MHC I found?

all nucleated cells

Where is MHC II found?

APCs - macrophages, dendritic cells, B cells

Example ligand of MHC I

viral peptides

Example ligand of MHC II

extracellular bacterial peptides

CD4+ IL-2 cytokine role in immune system

activates CD8+ killer T cells, which activate macrophages to defend against intracellular bacteria, virus, and cancer

Diseases related to IL-2 cytokine

multiple sclerosis, t1d

CD4+ IL-4 cytokine role in immune system

aid in differentiation and antibody production of B cells

Disease associated with IL-4 cytokine

asthma, allergic diseases

Vaccination purpose

mimics infection to train adaptive immunity, creating memory cells without causing disease

Result of defective B or T cells

increased susceptibility to infection

Result of loss of self-tolerance

autoimmunity; e.g., lupus, rheumatoid arthiritis

Examples of T and/or B cells targeting self-tissues

MS, Chron’s, T1D, RA

Hypersensitivity causes

overactive responses to harmless antigens caused when IgE binds to antigen resulting in immune response

Immunodeficiency causes

HIV destroys CD4+ T cells, crippling adaptive defense

Primary immunodeficiency

caused by genetic factors

Secondary immunodficiency

caused by external factors