Immunity and Immune Responses: Key Concepts in Microbiology

Immunity: Types and Historical Development

Overview of Immunity

Immunity refers to the body's ability to resist infection and disease. It is classified into two main types: innate immunity and adaptive immunity.

• Innate Immunity: Non-specific defenses present from birth; acts against any pathogen.

• Adaptive Immunity: Specific resistance developed in response to particular pathogens.

Historical Development

• Pasteur's Observations: Immunity in chickens injected with weakened pathogens.

• Von Behring: Nobel Prize for development of antitoxin.

• Milstein and Mitchell (1968): Identification of antibodies in serum.

Dual Nature of Adaptive Immunity

Cellular Origins

Adaptive immunity involves specialized cells derived from stem cells in the bone marrow:

• T cells and B cells are the primary lymphocytes responsible for adaptive immune responses.

The Nature of Antigens and Antibodies

Antigens

An antigen is any substance that elicits an immune response, typically by stimulating the production of specific antibodies or sensitized T cells.

• Epitopes (Antigenic Determinants): Specific regions on antigens recognized by antibodies.

• Hapten: Small molecule that becomes antigenic only when attached to a carrier molecule.

Antibodies (Immunoglobulins)

Antibodies are proteins called immunoglobulins that bind specifically to antigens. The number of antigen-binding sites determines the valence of the antibody.

Classes of Antibodies

Class	Structure	Location	Function	Serum %	Half-life
IgG	Monomer	Blood, lymph, intestine, placenta	Phagocytosis, neutralization, protection of fetus/newborn	80%	23 days
IgM	Pentamer	Blood, lymph, B cells	First Ab produced, agglutination	5-10%	5 days
IgA	Dimer	Secretions	Mucosal protection	10-15%	6 days
IgD	Monomer	B cells	Initiates immune response	0.2%	3 days
IgE	Monomer	Basophils, mast cells, blood	Allergic reactions, defense against parasites	0.002%	2 days

Antigen-Antibody Binding and Immune Reactions

Mechanisms of Action

• Agglutination: Clumping of antigens.

• Opsonization: Coating of antigens to enhance phagocytosis.

• Neutralization: Blocking of pathogen attachment.

• Antibody-dependent cell-mediated cytotoxicity (ADCC): Destruction of target cells by immune cells.

• Activation of complement: Cascade leading to pathogen lysis.

T-Dependent and T-Independent Antigens

T-Dependent Antigens

T-dependent antigens require the assistance of T cells to stimulate B cells for antibody production.

Major Histocompatibility Complex (MHC)

• MHC Class I: Present on all nucleated cells except red blood cells; present antigens to cytotoxic T cells.

• MHC Class II: Present on antigen-presenting cells (APCs) such as B cells, macrophages, dendritic cells; present antigens to helper T cells.

Roles of T Cells in Immunity

Helper T Cells (TH Cells)

• TH Cells (CD4+): Recognize antigens with MHC II; produce cytokines and differentiate into:

  • TH1: Activate macrophages and cytotoxic T cells

  • TH2: Activate B cells

  • TH Memory: Retain immunological memory

Cytotoxic T Cells (CTLs)

• CD8+ T Cells: Recognize antigens with MHC I; destroy infected or abnormal cells by releasing perforin and granzymes.

Regulatory T Cells (Treg)

• Suppress immune responses and prevent autoimmunity.

T Cell Maturation

• Occurs in the thymus; selection eliminates self-reactive T cells.

Natural Killer (NK) Cells and Cytokines

NK Cells

• Destroy cells lacking MHC I or expressing abnormal MHC I.

• Kill virus-infected and tumor cells.

Cytokines

• Chemical messengers that regulate immune responses.

• Include interleukins, interferons, and others.

Immunological Memory and Immunization

Primary and Secondary Immune Responses

• Primary Response: Initial antibody production after first exposure to antigen.

• Secondary (Memory) Response: Rapid and enhanced antibody production upon subsequent exposures.

Types of Immunity Acquisition

• Naturally Acquired Active Immunity: Infection-induced immunity.

• Naturally Acquired Passive Immunity: Transfer of antibodies (e.g., via colostrum).

• Artificially Acquired Active Immunity: Vaccination.

• Artificially Acquired Passive Immunity: Injection of antibodies.

Terminology of Adaptive Immunity

• Serology: Study of reactions between antibodies and antigens.

• Antiserum: Serum containing antibodies.

• Globulins: Serum proteins.

• Immunoglobulins: Antibodies.

• Gamma globulin: Serum fraction containing antibodies.

Example: Immunization

Vaccines stimulate artificially acquired active immunity by introducing antigens that prompt the body to produce its own antibodies and memory cells.

Additional info: Immunological memory is the basis for the effectiveness of vaccines and booster shots, which rely on the rapid secondary response to previously encountered antigens.