Clinical Pathology · MU Sofia

29. Immunity. Types of immunity. Pathological immune reactions. Pathomorphology.

Introduces immunity as innate and adaptive, covering non-specific barriers, phagocytes (neutrophils and macrophages) with the steps of phagocytosis, and NK cells. Goes on to humoral versus cell-mediated immunity and antibody structure, then the four hypersensitivity types with an ACID mnemonic and classic examples, autoimmunity with systemic and local diseases such as SLE, rheumatoid arthritis, Hashimoto thyroiditis and multiple sclerosis.

Inside: 1 section · 2 labelled pictures · 14 flashcards · 6 min

Topic 29 of 138 in the MU Sofia syllabus for Clinical Pathology (Medicine, Year 4). The free AceMed note covers it in 1 section, about 887 words (6 min of reading), with 14 flashcards.

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  1. Immunity. Types of immunity. Pathological immune reactions. Pathomorphology

Immunity. Types of immunity. Pathological immune reactions. Pathomorphology

Immunity is the state of an organism having the biological defenses to fight a disease. Immunity consists of the innate and adaptive immune system, these elements are nicely summed up below:

So, immunity can be innate or acquired, let's look at both.

Innate immunity

Innate (or nonspecific) immunity are the elements of the immune system that deal with pathogens non-specifically and are elements we are born with. These include external barricades such as skin and mucous membranes as well as internal defenses such as phagocytes, antimicrobial proteins and killer (NK) cells.

Unlike the adaptive immune system it does not confer long-lasting immunity to the host.

Barriers of non-specific immunity

The key barriers are that of skin and mucous membranes (barriers that line open cavities such as the nasopharynx and respiratory airways). There are also additional defense mechanisms such as sweating on the skin and tears in eyes that help to provide defense. The gastrointestinal tract includes elements such bile acid and gut flora that assist immunity.

Phagocytes

Phagocytes can be divided into two categories, Neutrophils and Macrophages (free and fixed respectively).

Neutrophils

  • Are formed from stem cells in bone marrow, most abundant form of white blood cell
  • Short-lived, highly motile and can enter parts of the tissue where other cells can’t
  • Phagocytic, nucleus divided into 2-5 lobes.
  • They operate at the beginning phase of inflammation often as a result of bacterial infection, migrating through blood vessels.
  • They are the predominant cells in pus

Macrophages

  • Large phagocytes, digesting pathogens as a secondary defense to neutrophils.
  • Found in all tissues, patrolling for pathogens through amoeboid (crawling) movement - we call them different things in different tissues
  • Do not get destroyed after phagocytosis, unlike neutrophils.

Steps of phagocytosis

  • Phagosome region is formed to ingest pathogen into cell
  • Fusion of lysosomes with phagosome creates a phagolysosome
  • Waste material then expelled

NK Cells

Natural killer cells are cytotoxic lymphocytes, central to the Innate immune system. They are responsible for dealing with viral-infected cells.

All healthy cells have a protein called MHC on their surface, when a cell is infected by a virus the MHC (Major Histocompatibility Complex) class I is reduced, so the NK cell is no longer inhibited and releases perforin and granzymes, causing lysis or apoptosis.

NK cells differentiate and mature in the bone marrow, lymph nodes, thymus and others.

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