Microbiology Chapters 18 & 19

SLCC Unit 3 Adaptive Specific Host Defenses & Disease of the Immune System

Specificity

Refers to the adaptive immune systems ability to target specific pathogens

Memory

Refers to its ability to quickly respond to pathogens to which it has previously been exposed

Primary Response

The initial adaptive immune response characterized by a lag phase, predominant IgM production, and formation of memory cells.

The first exposure to a pathogen or vaccine triggers this response

Secondary Response

A rapid, robust response upon re-exposure to an antigen, marked by high-affinity IgG production and memory cell activation.

“Subsequent exposures” result in a faster and stronger response as a result of the body’s memory of the first exposure.

Adaptive immunity

A specific immune response characterized by antigen specificity and memory

Antigen / Immunogen

Any substance that induces an immune response; Antigens are the target of that response

Epitope (Antigenic Determinant)

The specific part of an antigen recognized by immune receptors

Hapten

A small molecule that is antigenic only when attached to a carrier

MHC Class 1

Molecules found on all nucleated cells that present endogenous antigens to CD8+ T cells

“Cell that reports back” (monitor health of cells, detects infections or abnormal cells)

Signals to cytotoxic T cells to destroy the infected or abnormal cell.

MHC Class 2

Molecules expressed by professional antigen-presenting cells (APCs) that present exogenous antigens to CD4+ T cells

Activate the immune response to fight the invader.

APC (Antigen Presenting Cell)

Cells that process and present antigens via MHC 2

Present antigens specifically for the purpose of activating T cells

Macrophages/Dendritic cells

• Recognize pathogens through nonspecific signals/receptor interactions

• Phagocytosis, Digestion , presentation antigens on MHC2

B cells

• Recognize antigens that bind to antibody receptors

• Internalize the antigen via endocytosis, process it, and present it on MHC II molecules to activate T cells.

• This activation Enables conversion into memory and Plasma (Antibody secreting) B cells

T cells (T-Lymphocytes)

White blood cells involved in cell-mediated immunity: include CD4+ helper T cells, CD8+ cytotoxic T cells, and regulatory T cells

T Cell Maturation

Occurs in Thymus

Thymic Selection: Positive and Negative Selection

• Negative selection(Cortex) - Defective / non-functioning TCRs removed by apotosis(programmed controlled cell death)

• Positive selection(Cortex) - Thymocytes that will interact appropriately with MHC molecules survive

• (Cortex&Medulla)Negative selection to remove self-reacting thymocytes, those that react to self-antigens, by apoptosis. (central tolerance)

Peripheral Tolerance

• Second line of defense needed to protect against autoimmune disease. 

• Eliminating or inactivating self reactive T cells

• Anergy T cells cannot be fully activated

Helper T Cells (CD4)

Serve as the central orchestrators that help activate and direct functions of humoral and cellular immunity

• Characterized by the expression of CD4 on their surface

• Can only be activated by APCs presenting antigens associated with MHC II

• Enhance the pathogen-killing functions of macrophages and NK cells of innate immunity.

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Can only be activated by APCs presenting processed foreign epitopes in association with MHC II  and interaction of CD4 with the receptor complex.

Steps

• TCR recognizes the foreign epitopes

• CD4 binds to the MHC II-TCR complex for recognition.

• APC and T cell secrete cytokines to activate the helper T cell.

• Helper T cell proliferates, and naïve helper T cells differentiate into subtypes.

Cytotoxic T Cells (CD8) 

Primary effector cells for cellular immunity (“Kills Stuff”)

• Characterized by the expression of CD8.

• Recognize antigens presented in association with MHC I, either by APCs or by nucleated cells infected with an intracellular pathogen.

• Recognize and target cells that have been infected by intracellular pathogens, destroying infected cells along with the pathogens inside.

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Activation involves recognition of an antigen presented with MHC I and interaction of CD8 with the receptor complex.

Activated cytotoxic T cells can differentiate into effector cytotoxic T cells that target pathogens for destruction or memory cells that are ready to respond to subsequent exposures.

Steps

• TCR recognizes the foreign epitope.

• CD8 binds to the MHC I-TCR complex to recognize the foreign epitope.

• APC and T cell secrete cytokines to activate the cytotoxic T cell.

• TH1 cells, activated by the same epitope, release cytokines that promote CTL proliferation and differentiation.

Regulatory T Cells

To prevent undesirable and potentially damaging immune responses

• Characterized by the expression of CD4 on their surface

• Can only be activated by APCs presenting antigens associated with MHC II

T Cell Receptor (TCR) Structure

Involved in the first step of pathogen epitope recognition during the activation process.

Structure

• Variable region and a constant region, and the variable region provides the antigen-binding site the target of genetic rearrangement

• Smaller and less complex than the immunoglobulin molecules

• Two peptide chains (α and β chains)

B cells (B-Lymphocytes)

Cells that mediate humoral immunity by producing antibodies

Antibody

‘Y’-shaped proteins produced by B cells; composed of variable (FAB) and constant (Fc) regions